Add verbose logging for exopackage ABI calculation

Summary:
We're seeing unexpected abi misses for these rules.  Add some logging to
start tracking it down.

Test Plan:
Enabled these logs in my .bucklogging.properties.  Saw useful
information.
Improve error messages when build target parse preconditions fail

Summary:
When a build target pattern does not begin with '//', the build
fails with the error:

 java.lang.IllegalArgumentException: buildTargetPattern must start with //

Change this error message to include the name of the target that
caused the failure, making it easier to find and fix it.

Also, add quote marks in appropriate places in the error messages.

Test Plan:
1. Build Gerrit containing a broken maven_jar definition

 maven_jar(
   name = 'objenesis',
   id = 'org.objenesis:objenesis:1.2',
   sha1 = 'bfcb0539a071a4c5a30690388903ac48c0667f2a',
   license = 'DO_NOT_DISTRIBUTE',
   visibility = [':easymock', '//lib/powermock:powermock-reflect'],
   attach_source = False,
 )

2. Observe the error message:

 java.lang.IllegalArgumentException: ':easymock' must start with '//'

3. Don't spend ages tearing out hair trying to figure out where the
error is coming from.