[Fix #87] Include citekey in message on bibtex decoding error from repository.

Catches decoding error at databroker level to include citekey in message.
Could be improved by a better exception class for BibDecodingError.

pubs/databroker.py

@@ -45,7 +45,11 @@ class DataBroker(object):
 
     def pull_bibentry(self, citekey):
         bibdata_raw = self.filebroker.pull_bibfile(citekey)
-        return self.endecoder.decode_bibdata(bibdata_raw)
+        try:
+            return self.endecoder.decode_bibdata(bibdata_raw)
+        except self.endecoder.BibDecodingError as e:
+            e.message = "Unable to decode bibtex for paper {}.".format(citekey)
+            raise e
 
     def push_metadata(self, citekey, metadata):
         metadata_raw = self.endecoder.encode_metadata(metadata)