Fix crash on ERROR 1040 (HY000): Too many connections

If the max_connections are hit (system limit, not user limit),
mysql_conn crashes:

```
1> mysql_conn:start("localhost", 3306, "root", "pwd", "mysql", fun mysql:log/4, utf8, undefined).

=ERROR REPORT==== 25-Aug-2014::21:38:55 ===
Error in process <0.794.0> with exit value: {{badmatch,<<0 bytes>>},[{mysql_conn,greeting,2,[{file,"src/mysql_conn.erl"},{line,626}]},{mysql_conn,mysql_init,5,[{file,"src/mysql_conn.erl"},{line,588}]},{mysql_conn,init,9,[{file,"src/mysql_conn.erl"},{line,325}]}]}

{error,"timed out"}
2>
```

With this fix, the correct error is returned (tested with MySQL 5.5.37):

```
3> mysql_conn:start("localhost", 3306, "root", "pwd", "mysql", fun mysql:log/4, utf8, undefined).
mysql_conn:589: init error 1040: "Too many connections"
{error,login_failed}
mysql_recv:143: mysql_recv: Socket #Port<0.806> closed
4>
```