BUG-1070: introduce YangSyntaxErrorException

This introduces a proper checked exception, which is trown when a
syntactic error is encountered during parsing of the YANG module text.

Change-Id: If7445edce99d60306bad98e61b34d09c5bf06eb1
Signed-off-by: Robert Varga <rovarga@cisco.com>
Merge "BUG-1070: introduce YangSyntaxErrorException"