Fix TypeError in parser wrapper logic

In attempting to pass-through the string representation of an exception
we are wrapping, we made the erroneous assumption that `args[0]` would
always be a string (or something that can concatenate cleanly with a
string). This turns out not to be the case with `IllegalMonthError`,
where it is an integer, so to avoid raising an erroneous `TypeError`, we
first convert the wrapped exception to a string.

See GH issue dateutil#981.

changelog.d/987.bugfix.rst

@@ -0,0 +1,4 @@
+Fixed a bug in the parser where non-``ValueError`` exceptions would be raised
+during exception handling; this would happen, for example, if an
+``IllegalMonthError`` was raised in ``dateutil`` code. Fixed by Mark Bailey.
+(gh issue #981, pr #987).

dateutil/parser/_parser.py

@@ -654,7 +654,7 @@ def parse(self, timestr, default=None,
         try:
             ret = self._build_naive(res, default)
         except ValueError as e:
-            six.raise_from(ParserError(e.args[0] + ": %s", timestr), e)
+            six.raise_from(ParserError(str(e) + ": %s", timestr), e)
 
         if not ignoretz:
             ret = self._build_tzaware(ret, res, tzinfos)

dateutil/test/test_parser.py

@@ -743,6 +743,10 @@ def test_out_of_bound_day(self):
         with pytest.raises(ParserError):
             parse("Feb 30, 2007")
 
+    def test_illegal_month_error(self):
+        with pytest.raises(ParserError):
+            parse("0-100")
+
     def test_day_sanity(self, fuzzy):
         dstr = "2014-15-25"
         with pytest.raises(ParserError):