adjust message for status_code based exceptions

from: 5xx HTTP Error: Reason
to  : 5xx: Reason

The "HTTP Error" part was in there to emulate Request's error messages
from response.raise_for_status(), but it reads a lot better without.

gallery_dl/extractor/common.py

@@ -74,7 +74,7 @@ class Extractor():
                         response.encoding = encoding
                     return response
 
-                msg = "{} HTTP Error: {} for url: {}".format(
+                msg = "{}: {} for url: {}".format(
                     code, response.reason, url)
                 if code < 500 and code != 429:
                     break

test/test_results.py

@@ -72,7 +72,7 @@ class TestExtractorResults(unittest.TestCase):
         except exception.StopExtraction:
             pass
         except exception.HttpError as exc:
-            if re.match(r"5\d\d HTTP Error:", str(exc)):
+            if re.match(r"5\d\d: ", str(exc)):
                 self.skipTest(exc)
             raise