Improve Java wrapping library loading error reporting

Rather than swallowing UnsatisfiedLinkErrors print the
stack trace to stderr. If explicitly loading a library
using a full path first check if file exists, only try
to load if it exists.

Change-Id: I67066e1c9377c0421bac73d0c0a769cc45300e51
Merge topic 'java-native-lib-loading' into master

e748e467 Improve Java wrapping library loading error reporting