Error when source name is not found instead of "0 packages found"

~~~sh
> choco search -s non_existent_source
Chocolatey v0.10.3
0 packages found.
~~~

Should error instead, similar to how it errors when you try to reach a source by url.

It should look something like this:
`Invalid URI: The hostname could not be parsed.`

Reproduce `choco search -s non_existent_source` and compare its source-name handling with the existing URL-source error path. Done means an invalid source name reports an error such as `Invalid URI: The hostname could not be parsed.` instead of `0 packages found.`

Written by the indexing model from the issue text.