Added some robustness to the stasistics code to handle corrupt files.

Added some robustness to the stasistics code to handle corrupt files.
The boolean parameter errors_return_empty can be passed to a number
of the methods in Statdb; if errors_return_empty is true, then they
will not raise exceptions, but instead return empty or zero values.

Created a ParseError exception which should be raised by parsers which
run into parsing problems. So far, only pypo.py does this (and in a
somewhat rudimentary way).

git-svn-id: http://translate.svn.sourceforge.net/svnroot/translate/src/trunk/translate@7646 54714841-351b-0410-a198-e36a94b762f5