Add simple stub error support in Zinc Java analysis #149

Currently, errors for classfiles that are missing (generally, ClassNotFoundException) from the classpath during javac analysis do not include enough information about the path taken to reach the missing classfile.

Rather than treating this as an unlikely/unexpected error, ideally this should result in something like:

While analyzing com.a.A, failed to locate: `com.b.B`.
This class must be present in order to apply dependency tracking.