Improve error message when build fails because of missing metadata
Gradle 6.0 removed the "artifact" metadata source by default.
This means that if a module is published _only_ with an artifact,
previous version of Gradle would find it, but 6.0 would fail with
a module missing exception.
The problem is that it's hard to realize that the issue comes
from the change of this default artifact sources.
This commit tries to improve the situation by recognizing that
a failure is related to not finding metadata, and in this case
would suggest that if the metadata is missing, it is still
possible that the jar is present.
The drawback of this approach is that we're unsure: if, for
some reason, the module is _really_ absent, then we gave a
wrong advice. This means, in particular, in case of wrong
coordinates.
