[MRESOLVER-335] Proposal for better errors

[cstamas]

Basically drop the method building text message (and build something very basic instead). It is the application that should build proper output, as it does have access to org.eclipse.aether.resolution.ArtifactResolutionException#getResults and can and should build meaningful error instead.

This PR does not fixes the issue for Maven, it merely does these things:

• alters the message built by ArtifactResolutionException that does lack information to be complete
• but IMHO Maven is not meant to use this exception message as is (as it happens today), instead
• this PR makes possible that in Maven code build proper message, as it now exposes everything (maps remote repository to exception)

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https://issues.apache.org/jira/browse/MRESOLVER-335

[cstamas]

@slawekjaranowski as reporter, anything to add?

[cstamas]

One thing am unsure about: Map<ArtifactRepository, Exception> exceptions in ArtifactResult... I think it should be Map<ArtifactRepository, List<Exception>> exceptions instead...

[cstamas]

Reworked it and is much simpler change (and is even backward compat, so no tampering with japicm needed)