Improve plugin not found error message

### What feature do you want to see added?

When plugin is not found in an update center, the error message is lengthy and not quite concise.

Presuming a `plugins.txt` entry of `break-it:4.2`, this is the error:
```
...
Couldn't find checksum for: break-it

Resolving dependencies of break-it by downloading plugin to temp file /tmp/break-it1927681041617501377.jpi and parsing MANIFEST.MF
Will use url: https://INTERNAL_UC/download/plugins/break-it/4.2/break-it.hpi to download break-it plugin
Unable to resolve plugin URL https://INTERNAL_UC/download/plugins/break-it/4.2/break-it.hpi, or download plugin break-it to file: status code: 404, reason phrase: Not Found
Unable to resolve plugin URL https://INTERNAL_UC/download/plugins/break-it/4.2/break-it.hpi, or download plugin break-it to file: status code: 404, reason phrase: Not Found
Unable to resolve plugin URL https://INTERNAL_UC/download/plugins/break-it/4.2/break-it.hpi, or download plugin break-it to file: status code: 404, reason phrase: Not Found
java.io.IOException: Unable to resolve plugin URL https://INTERNAL_UC/download/plugins/break-it/4.2/break-it.hpi, or download plugin break-it to file: status code: 404, reason phrase: Not Found
at io.jenkins.tools.pluginmanager.impl.PluginManager.getViaHttpWithResponseHandler(PluginManager.java:1368)
at io.jenkins.tools.pluginmanager.impl.PluginManager.downloadHttpToFile(PluginManager.java:1327)
at io.jenkins.tools.pluginmanager.impl.PluginManager.downloadToFile(PluginManager.java:1282)
at io.jenkins.tools.pluginmanager.impl.PluginManager.downloadToFile(PluginManager.java:1255)
at io.jenkins.tools.pluginmanager.impl.PluginManager.downloadPlugin(PluginManager.java:1193)
at io.jenkins.tools.pluginmanager.impl.PluginManager.resolveDependenciesFromManifest(PluginManager.java:943)
at io.jenkins.tools.pluginmanager.impl.PluginManager.resolveDirectDependencies(PluginManager.java:1061)
at io.jenkins.tools.pluginmanager.impl.PluginManager.resolveRecursiveDependencies(PluginManager.java:1095)
at io.jenkins.tools.pluginmanager.impl.PluginManager.findPluginsAndDependencies(PluginManager.java:692)
at io.jenkins.tools.pluginmanager.impl.PluginManager.start(PluginManager.java:232)
at io.jenkins.tools.pluginmanager.impl.PluginManager.start(PluginManager.java:189)
at io.jenkins.tools.pluginmanager.cli.Main.main(Main.java:52)
Caused by: org.apache.http.client.HttpResponseException: status code: 404, reason phrase: Not Found
at org.apache.http.impl.client.AbstractResponseHandler.handleResponse(AbstractResponseHandler.java:70)
at org.apache.http.impl.client.CloseableHttpClient.execute(CloseableHttpClient.java:223)
at org.apache.http.impl.client.CloseableHttpClient.execute(CloseableHttpClient.java:165)
at io.jenkins.tools.pluginmanager.impl.PluginManager.getViaHttpWithResponseHandler(PluginManager.java:1360)
... 11 more
Downloading from mirrors failed, falling back to https://archives.jenkins.io/
Unable to resolve plugin URL https://archives.jenkins.io/plugins/break-it/4.2/break-it.hpi, or download plugin break-it to file: status code: 404, reason phrase: Not Found
java.io.IOException: Unable to resolve plugin URL https://archives.jenkins.io/plugins/break-it/4.2/break-it.hpi, or download plugin break-it to file: status code: 404, reason phrase: Not Found
at io.jenkins.tools.pluginmanager.impl.PluginManager.getViaHttpWithResponseHandler(PluginManager.java:1368)
at io.jenkins.tools.pluginmanager.impl.PluginManager.downloadHttpToFile(PluginManager.java:1327)
at io.jenkins.tools.pluginmanager.impl.PluginManager.downloadToFile(PluginManager.java:1282)
at io.jenkins.tools.pluginmanager.impl.PluginManager.downloadToFile(PluginManager.java:1288)
at io.jenkins.tools.pluginmanager.impl.PluginManager.downloadToFile(PluginManager.java:1255)
at io.jenkins.tools.pluginmanager.impl.PluginManager.downloadPlugin(PluginManager.java:1193)
at io.jenkins.tools.pluginmanager.impl.PluginManager.resolveDependenciesFromManifest(PluginManager.java:943)
at io.jenkins.tools.pluginmanager.impl.PluginManager.resolveDirectDependencies(PluginManager.java:1061)
at io.jenkins.tools.pluginmanager.impl.PluginManager.resolveRecursiveDependencies(PluginManager.java:1095)
at io.jenkins.tools.pluginmanager.impl.PluginManager.findPluginsAndDependencies(PluginManager.java:692)
at io.jenkins.tools.pluginmanager.impl.PluginManager.start(PluginManager.java:232)
at io.jenkins.tools.pluginmanager.impl.PluginManager.start(PluginManager.java:189)
at io.jenkins.tools.pluginmanager.cli.Main.main(Main.java:52)
Caused by: org.apache.http.client.HttpResponseException: status code: 404, reason phrase: Not Found
at org.apache.http.impl.client.AbstractResponseHandler.handleResponse(AbstractResponseHandler.java:70)
at org.apache.http.impl.client.CloseableHttpClient.execute(CloseableHttpClient.java:223)
at org.apache.http.impl.client.CloseableHttpClient.execute(CloseableHttpClient.java:165)
at io.jenkins.tools.pluginmanager.impl.PluginManager.getViaHttpWithResponseHandler(PluginManager.java:1360)
... 12 more
Couldn't find checksum for: break-it

[Removed 1700 lines]

Security warnings:
io.jenkins.tools.pluginmanager.impl.AggregatePluginPrerequisitesNotMetException: Plugin prerequisite not met:
Plugin break-it:4.2 has unresolvable dependencies: Unable to resolve dependencies for break-it
at io.jenkins.tools.pluginmanager.impl.PluginManager.start(PluginManager.java:240)
at io.jenkins.tools.pluginmanager.impl.PluginManager.start(PluginManager.java:189)
at io.jenkins.tools.pluginmanager.cli.Main.main(Main.java:52)
Suppressed: io.jenkins.tools.pluginmanager.impl.PluginDependencyException: Plugin break-it:4.2 has unresolvable dependencies: Unable to resolve dependencies for break-it
at io.jenkins.tools.pluginmanager.impl.PluginManager.resolveRecursiveDependencies(PluginManager.java:1099)
at io.jenkins.tools.pluginmanager.impl.PluginManager.findPluginsAndDependencies(PluginManager.java:692)
at io.jenkins.tools.pluginmanager.impl.PluginManager.start(PluginManager.java:232)
... 2 more
Caused by: io.jenkins.tools.pluginmanager.impl.DownloadPluginException: Unable to resolve dependencies for break-it
at io.jenkins.tools.pluginmanager.impl.PluginManager.resolveDependenciesFromManifest(PluginManager.java:945)
at io.jenkins.tools.pluginmanager.impl.PluginManager.resolveDirectDependencies(PluginManager.java:1061)
at io.jenkins.tools.pluginmanager.impl.PluginManager.resolveRecursiveDependencies(PluginManager.java:1095)
... 4 more
Plugin prerequisite not met:
Plugin break-it:4.2 has unresolvable dependencies: Unable to resolve dependencies for break-it
...
```

### Upstream changes

Abort download right away with a message clearly indicating the plugin name was not found.