Revert error packet parsing changes (#508)

Revert the changes made in commit 02910b7 to the mysql error packet
parsing, as it didn't take the CLIENT_PROTOCOL_41 case into account.

It appears that the non-CLIENT_PROTOCOL_41 case was off by one as well
(sliced data on 4 vs 3).

I also added two new tests to cover those cases.

See: https://dev.mysql.com/doc/internals/en/packet-ERR_Packet.html

pymysql/err.py

@@ -97,6 +97,11 @@ del _map_error, ER
 
 def raise_mysql_exception(data):
     errno = struct.unpack('<h', data[1:3])[0]
-    errval = data[4:].decode('utf-8', 'replace')
+    is_41 = data[3:4] == b"#"
+    if is_41:
+        # client protocol 4.1
+        errval = data[9:].decode('utf-8', 'replace')
+    else:
+        errval = data[3:].decode('utf-8', 'replace')
     errorclass = error_map.get(errno, InternalError)
     raise errorclass(errno, errval)

pymysql/tests/__init__.py

@@ -5,6 +5,7 @@ from pymysql.tests.test_basic import *
 from pymysql.tests.test_connection import *
 from pymysql.tests.test_converters import *
 from pymysql.tests.test_cursor import *
+from pymysql.tests.test_err import *
 from pymysql.tests.test_issues import *
 from pymysql.tests.test_load_local import *
 from pymysql.tests.test_nextset import *

pymysql/tests/test_err.py

@@ -0,0 +1,21 @@
+import unittest2
+
+from pymysql import err
+
+
+__all__ = ["TestRaiseException"]
+
+
+class TestRaiseException(unittest2.TestCase):
+
+    def test_raise_mysql_exception(self):
+        data = b"\xff\x15\x04Access denied"
+        with self.assertRaises(err.OperationalError) as cm:
+            err.raise_mysql_exception(data)
+        self.assertEqual(cm.exception.args, (1045, 'Access denied'))
+
+    def test_raise_mysql_exception_client_protocol_41(self):
+        data = b"\xff\x15\x04#28000Access denied"
+        with self.assertRaises(err.OperationalError) as cm:
+            err.raise_mysql_exception(data)
+        self.assertEqual(cm.exception.args, (1045, 'Access denied'))