BUG-7267: catch RuntimeExceptions when processing sources

BUG-7267: catch RuntimeExceptions when processing sources

This adds wrapping of all RuntimeExceptions when building
EffectiveSchemaContext. This will allow users to identify
the offending source.

Since raw RuntimeExceptions should not be happening, but
rather should be specialized to SourceException and its
subclasses, also emit a warning guiding users to file
issues to fix codepaths which do not do so.

Also fixes up some instances where we use checkArgument()
and we ca actually use SourceException.throwIf().

Change-Id: Ie1c3d05b39b251996b746c38c5b7a51946b19ff5
Signed-off-by: Robert Varga <rovarga@cisco.com>