RFR: JDK-8215991: Stop hiding exception from ArtifactResolver failures in tests

[Erik Joelsson]

In the test lib, we have an ArtifactResolver which is used by tests that 
need external resources. Internally at Oracle we hook Jib into this to 
provide such dependencies. For other users, it's possible to specify the 
path to such a dependency using a system property.

If something goes wrong while resolving the dependencies, the 
ArtifactResolver currently swallows the exception and falls back on the 
default property based resolver, making it hard to figure out the problem.

If the user provided the environment for calling Jib, I don't think we 
should try to revert to the default resolver on exception, but instead 
the exception should be propagated to the test.

Webrev: http://cr.openjdk.java.net/~erikj/8215991/webrev.01/

Bug: https://bugs.openjdk.java.net/browse/JDK-8215991

/Erik