ENH: Provide more informative message whenever using older citeproc w…

…ithout encoding arg

Elderly me forgot details of the issue since it happened years ago.
So to get better idea when things fail - provide information that it
is due to outdated citeproc

duecredit/io.py

@@ -28,6 +28,7 @@
 from .config import CACHE_DIR, DUECREDIT_FILE
 from .entries import BibTeX, Doi, Text, Url
 from .log import lgr
+from .versions import external_versions
 
 _PREFERRED_ENCODING = locale.getpreferredencoding()
 platform_system = platform.system().lower()
@@ -310,8 +311,16 @@ def format_bibtex(bibtex_entry, style='harvard1'):
                 # we should be able to provide encoding argument
                 bib_source = cpBibTeX(fname, encoding='utf-8')
         except Exception as e:
-            lgr.error("Failed to process BibTeX file %s: %s" % (fname, e))
-            return "ERRORED: %s" % str(e)
+            msg = "Failed to process BibTeX file %s: %s." % (fname, e)
+            citeproc_version = external_versions['citeproc']
+            if 'unexpected keyword argument' in str(e) and \
+                    citeproc_version and citeproc_version < '0.4':
+                err = "need a newer citeproc-py >= 0.4.0"
+                msg += " You might just " + err
+            else:
+                err = str(e)
+            lgr.error(msg)
+            return "ERRORED: %s" % err
         finally:
             # return warnings back
             warnings.filters = old_filters