_mssql.pyx: Prevent error string growing

with repeated failed connection attempts.

Fixes GH-145

Cc: @sontek

```bash
(py26.venv)marca@marca-mac2:~/dev/git-repos/pymssql$ python stuff/test_pymssql_connect_failure.py
Exception str:
     DB-Lib error message 20017, severity 9:
    Unexpected EOF from the server
     DB-Lib error message 20002, severity 9:
    Adaptive Server connection failed

Exception str:
     DB-Lib error message 20017, severity 9:
    Unexpected EOF from the server
     DB-Lib error message 20002, severity 9:
    Adaptive Server connection failed

Exception str:
     DB-Lib error message 20017, severity 9:
    Unexpected EOF from the server
     DB-Lib error message 20002, severity 9:
    Adaptive Server connection failed

Exception str:
     DB-Lib error message 20017, severity 9:
    Unexpected EOF from the server
     DB-Lib error message 20002, severity 9:
    Adaptive Server connection failed

Exception str:
     DB-Lib error message 20017, severity 9:
    Unexpected EOF from the server
     DB-Lib error message 20002, severity 9:
    Adaptive Server connection failed
```