Support Coverage reporting in gpodder.unittests

If "python-coverage" is installed, report the test
coverage for all currently-tested modules in gPodder
while doing the unittest with "make unittest".

src/gpodder/unittests.py

@@ -25,20 +25,47 @@
 import doctest
 import unittest
 import gettext
+import sys
 
 # Which package and which modules in the package should be tested?
 package = 'gpodder'
 modules = ['util', 'libtagupdate']
+coverage_modules = []
 
 suite = unittest.TestSuite()
 
 for module in modules:
     m = __import__('.'.join((package, module)), fromlist=[module])
+    coverage_modules.append(m)
     # Emulate a globally-installed no-op gettext _() function
     if not hasattr(m, '_'):
         setattr(m, '_', lambda x: x)
     suite.addTest(doctest.DocTestSuite(m))
 
 runner = unittest.TextTestRunner(verbosity=2)
-runner.run(suite)
+
+try:
+    import coverage
+except ImportError:
+    coverage = None
+
+if coverage is not None:
+    coverage.erase()
+    coverage.start()
+
+result = runner.run(suite)
+
+if not result.wasSuccessful():
+    sys.exit(1)
+
+if coverage is not None:
+    coverage.stop()
+    coverage.report(coverage_modules)
+    coverage.erase()
+
+if coverage is None:
+    print >>sys.stderr, """
+    No coverage reporting done (Python module "coverage" is missing)
+    Please install the python-coverage package to get coverage reporting.
+    """