Fixed PHP-617: Use real error codes for MongoConnectionException on ctor failure.

Not much to explain, except that I have changed the notion that an empty option
name or value is not an error.  The spec at
https://wiki.10gen.com/display/10GEN/Connection+String+Format doesn't define
any such options, nor makes comments about this.
Fixed passing error messages onwards.
Fixed code comment.
Merged pull request #248