RFR: JDK-8170392: JDK-8031567 broke builds from source bundles

[Erik Joelsson]

In JDK-8031567, the format for storing source revision information for 
use in builds from source bundles changed. The behavior when building 
without either mercurial or the stored revision information also 
changed. We used to silently ignore the lack of source revision 
information, but now the build fails.

Since it's not an uncommon use case in our Oracle internal 
infrastructure (JPRT, Mach 5) to have builds without either mercurial or 
stored revision information, this needs to be downgraded this to at most 
a warning.

Bug: https://bugs.openjdk.java.net/browse/JDK-8170392

Webrev: http://cr.openjdk.java.net/~erikj/8170392/webrev.01/

/Erik