Bug 1326250 - Fix TypeError in PerformanceParser error logging (#2868)

Previously schema violations would result in:
`TypeError: not enough arguments for format string`

The exception `message` property is used instead of the `str`
representation, since the latter results in pages of log output
rather than just the human readable message:
https://github.com/Julian/jsonschema/blob/v2.6.0/jsonschema/exceptions.py#L59-L88
https://python-jsonschema.readthedocs.io/en/v2.6.0/errors/#jsonschema.exceptions.ValidationError.message

treeherder/log_parser/parsers.py

@@ -494,9 +494,8 @@ class PerformanceParser(ParserBase):
             except ValueError:
                 logger.warning("Unable to parse Perfherder data from line: %s",
                                line)
-            except jsonschema.ValidationError:
+            except jsonschema.ValidationError as e:
                 logger.warning("Perfherder line '%s' does not comply with "
-                               "json schema: %s", line)
-            # don't mark as complete, in case there are multiple performance
-            # artifacts
-            # self.complete = True
+                               "json schema: %s", line, e.message)
+
+            # Don't mark the parser as complete, in case there are multiple performance artifacts.