qbrowse: (internally handled) traceback if revno=""

[Kuno Meyer]

1. "bzr qbrowse"
2. clear the "revision" field, press ShowRevision
3. see console output:

Traceback (most recent call last):
  File "C:/Documents and Settings/xkm/Application Data/bazaar/2.0/plugins\qbzr\browse.py", line 184, in reload_tree
    revspec = RevisionSpec.from_string(unicode(self.revision_edit.text()))
  File "bzrlib\revisionspec.pyc", line 181, in from_string
bzrlib.errors.NoSuchRevisionSpec: No namespace registered for string: u''

Possible improvements (just variants):
- provide a MessageBox-Feedback that the operation failed
- or: disply "Show revision" grayed as long as the revision number is not valid
- or: treat Revno="" as an alias for the latest revision

in addition:
- make "Show Revision" the default button when editing the revno filed, thus pressing <RETURN> should work

[Lukáš Lalinský (luks)]

It actualy already does the first option, but only for invalid revision specifiers. I didn't realize it can raise also NoSuchRevisionSpec. I'll add that and aliasing "" to the latest revision.