crash with bibtex errors with no line number

[Emmanuel Beffara]

When bibtex reports an error with no line number, typically something like

  I found no \citation commands---while reading file foo.aux

rubber crashes:

  $ rubber -d foo.tex
  compiling foo.tex...
  running: bibtex foo.aux...
  [biblio] There were errors running bibtex.
  Traceback (most recent call last):
    File "/usr/local/bin/rubber", line 17, in <module>
      sys.exit (cmdline (args))
    File "/usr/local/lib/python2.7/site-packages/rubber/cmdline.py", line 432, in __call__
      self.main (cmdline)
    File "/usr/local/lib/python2.7/site-packages/rubber/cmdline.py", line 373, in main
      self.process_source (env)
    File "/usr/local/lib/python2.7/site-packages/rubber/cmdline.py", line 460, in process_source
      self.build (env)
    File "/usr/local/lib/python2.7/site-packages/rubber/cmdline.py", line 406, in build
      for err in env.final.failed().get_errors():
    File "/usr/local/lib/python2.7/site-packages/rubber/biblio.py", line 84, in get_errors
      line = int (m.group ("line"))
  TypeError: int() argument must be a string or a number, not 'NoneType'

because the regexp group "line" is undefined for this message. The attached patch fixes the issue.

[Sebastian Kapfer (caci)]

Committed a regression test. Thanks!