remove unnecessary try catch blocks

https://github.com/owncloud/core/pull/3654 added global exception handling. This PR just removes the try catch in db.php for the PDO connection and checks the MDB2 connection with `self::raiseExceptionOnError( self::$MDB2 );`

@DeepDiver1975 @icewind1991 @MTGap please test

The other changes are whitespace fixes. @DeepDiver1975 have you meanwhile configured your IDE to using tabs?