[SUREFIRE-1671] Improve error message in case dependency cannot be downloaded

**[Konrad Windszus](https://issues.apache.org/jira/secure/ViewProfile.jspa?name=kwin)** opened **[SUREFIRE-1671](https://issues.apache.org/jira/browse/SUREFIRE-1671?redirect=false)** and commented

In case a surefire provider dependency cannot be downloaded for some reason the only error message being emitted is a ClassNotFoundException. Instead a proper error message stating which dependency cannot be downloaded for which reason should be given out.

This is the example log of a build where surefire-junit4-3.0.0-M3.jar could not be downloaded.

```
[INFO] --- maven-surefire-plugin:3.0.0-M3:test (default-test) @ org.apache.sling.models.impl ---
Downloading from nexus: /org/apache/maven/surefire/surefire-junit4/3.0.0-M3/surefire-junit4-3.0.0-M3.jar
[INFO]
[INFO] -------------------------------------------------------
[INFO] T E S T S
[INFO] -------------------------------------------------------
[INFO]
[INFO] Results:
[INFO]
[INFO] Tests run: 0, Failures: 0, Errors: 0, Skipped: 0
[INFO]
[INFO] ------------------------------------------------------------------------
[INFO] BUILD FAILURE
[INFO] ------------------------------------------------------------------------
[INFO] Total time: 4.878 s
[INFO] Finished at: 2019-06-05T08:13:16+02:00
[INFO] ------------------------------------------------------------------------
[ERROR] Failed to execute goal org.apache.maven.plugins:maven-surefire-plugin:3.0.0-M3:test (default-test) on project org.apache.sling.models.impl: There are test failures.
[ERROR]
[ERROR] Please refer to /Users/konradwindszus/git/sling-org-apache-sling-models-impl/target/surefire-reports for the individual test results.
[ERROR] Please refer to dump files (if any exist) [date].dump, [date]-jvmRun[N].dump and [date].dumpstream.
[ERROR] There was an error in the forked process
[ERROR] java.lang.ClassNotFoundException: org.apache.maven.surefire.junit4.JUnit4Provider
[ERROR] org.apache.maven.surefire.booter.SurefireBooterForkException: There was an error in the forked process
[ERROR] java.lang.ClassNotFoundException: org.apache.maven.surefire.junit4.JUnit4Provider
[ERROR] at org.apache.maven.plugin.surefire.booterclient.ForkStarter.fork(ForkStarter.java:657)
[ERROR] at org.apache.maven.plugin.surefire.booterclient.ForkStarter.run(ForkStarter.java:283)
[ERROR] at org.apache.maven.plugin.surefire.booterclient.ForkStarter.run(ForkStarter.java:246)
[ERROR] at org.apache.maven.plugin.surefire.AbstractSurefireMojo.executeProvider(AbstractSurefireMojo.java:1161)
[ERROR] at org.apache.maven.plugin.surefire.AbstractSurefireMojo.executeAfterPreconditionsChecked(AbstractSurefireMojo.java:1002)
[ERROR] at org.apache.maven.plugin.surefire.AbstractSurefireMojo.execute(AbstractSurefireMojo.java:848)
[ERROR] at org.apache.maven.plugin.DefaultBuildPluginManager.executeMojo(DefaultBuildPluginManager.java:137)
[ERROR] at org.apache.maven.lifecycle.internal.MojoExecutor.execute(MojoExecutor.java:210)
[ERROR] at org.apache.maven.lifecycle.internal.MojoExecutor.execute(MojoExecutor.java:156)
[ERROR] at org.apache.maven.lifecycle.internal.MojoExecutor.execute(MojoExecutor.java:148)
[ERROR] at org.apache.maven.lifecycle.internal.LifecycleModuleBuilder.buildProject(LifecycleModuleBuilder.java:117)
[ERROR] at org.apache.maven.lifecycle.internal.LifecycleModuleBuilder.buildProject(LifecycleModuleBuilder.java:81)
[ERROR] at org.apache.maven.lifecycle.internal.builder.singlethreaded.SingleThreadedBuilder.build(SingleThreadedBuilder.java:56)
[ERROR] at org.apache.maven.lifecycle.internal.LifecycleStarter.execute(LifecycleStarter.java:128)
[ERROR] at org.apache.maven.DefaultMaven.doExecute(DefaultMaven.java:305)
[ERROR] at org.apache.maven.DefaultMaven.doExecute(DefaultMaven.java:192)
[ERROR] at org.apache.maven.DefaultMaven.execute(DefaultMaven.java:105)
[ERROR] at org.apache.maven.cli.MavenCli.execute(MavenCli.java:956)
[ERROR] at org.apache.maven.cli.MavenCli.doMain(MavenCli.java:288)
[ERROR] at org.apache.maven.cli.MavenCli.main(MavenCli.java:192)
[ERROR] at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
[ERROR] at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
[ERROR] at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
[ERROR] at java.lang.reflect.Method.invoke(Method.java:498)
[ERROR] at org.codehaus.plexus.classworlds.launcher.Launcher.launchEnhanced(Launcher.java:282)
[ERROR] at org.codehaus.plexus.classworlds.launcher.Launcher.launch(Launcher.java:225)
[ERROR] at org.codehaus.plexus.classworlds.launcher.Launcher.mainWithExitCode(Launcher.java:406)
[ERROR] at org.codehaus.plexus.classworlds.launcher.Launcher.main(Launcher.java:347)
[ERROR]
[ERROR] -> [Help 1]
[ERROR]
[ERROR] To see the full stack trace of the errors, re-run Maven with the -e switch.
[ERROR] Re-run Maven using the -X switch to enable full debug logging.
[ERROR]
[ERROR] For more information about the errors and possible solutions, please read the following articles:
[ERROR] [Help 1] http://cwiki.apache.org/confluence/display/MAVEN/MojoExecutionException
```

---

**Affects:** 3.0.0-M3

Start by tracing the provider-loading failure around ForkStarter and SurefireBooterForkException, using the supplied log as the reproduction scenario. Follow how the surefire-junit4 dependency failure becomes a ClassNotFoundException; done means the error identifies the dependency and explains the download reason instead.

Written by the indexing model from the issue text.