SongUsage isn't helpful when saving reports

Two things:
Firstly the songusage report form doesn't make it explicit that it only wants a directory in the box and not a filename.

Secondly if the directory is entered instead of chosen giving rise to misspelt or non-existant entries we don't mention this to the user we just log an exception and the screen goes away having not saved any report and not told the user why we won't save a report.

Exception in log:

2011-01-27 20:44:59,694 openlp.plugins.songusage.forms.songusagedetailform ERROR Failed to write out song usage records
Traceback (most recent call last):
  File "/home/jt/projects/openlp/openlp/trivialfixes/openlp/plugins/songusage/forms/songusagedetailform.py", line 88, in accept
    file = open(outname, u'w')
IOError: [Errno 2] No such file or directory: u'~/test_report/usage_detail_01012011_31012011.txt'
2011-01-27 20:46:03,913 openlp.plugins.songusage.forms.songusagedetailform DEBUG Detailed report generated

Note the ridiculousness of the last debug entry too.