Refactor exceptions

This should be done in the master branch.

Another solution, avoiding to create many classes, would be to use the exception code, which is always kept as 0 currently (the default value of the Exception class). You could have a code for each case (with constants in the DBALException class) and then checking $e->getCode() to identify what went wrong.

I'd prefer actual named exceptions. It makes catching them simpler. However, adding some code defined in DBAL would be an acceptable alternative.

try {
    /* ... /*
} catch (NoSuchTableException $e) {
    // do something
} catch (DuplicateKeyException $e) {
    // do something else
}

v.s.

try {
    /* ... /*
} catch (DBALException $e) {
    if ($e->getCode() == DBALException::NO_SUCH_TABLE) {
        // do something
    } elseif ($e->getCode() == DBALException::DUPLICATE_KEY) {
        // do something else
    } else {
        throw $e;
    }
}

I would also prefer named exceptions. You're going to have a lot of problems providing the "code" value in DBALException in any case: SQLSTATE codes are alphanumeric, and will cause warnings/errors when creating new exception.

Besides we can get the SQL state code now:

    try {
        // ...
    } catch (\Doctrine\DBAL\DBALException $e) {
        $code = $e->getPrevious()->getCode();
        // do stuff with $code
    }

The problem is that there are a lot of error codes defined in the ANSI SQL standard: http://www.postgresql.org/docs/9.2/static/errcodes-appendix.html

Maybe throwing an specific exception for each "class" of SQLSTATE codes? So if the error code from a PDO exception starts with 23, DBAL would throw `\Doctrine\DBAL\Exception\IntegrityConstraintViolationException`.

This also seems like the logic to handle throwing exceptions should be contained in the platforms as some implementations may differ. You could have a method in `AbstractPlatform` that takes care of the ANSI SQLSTATE error code classes and leave it up subclasses to deal with platform specific cases. Whenever `Connection` catches a `PDOException`, dispatch it to the platform to deal with.

Example: https://gist.github.com/chrisguitarguy/e021918900e93dca304d

Thoughts?

I have implemented a thing of that kind in a personal project (on top of Doctrine). It is really useful to be able to catch a ForeignKeyViolationException, and get with entity/field caused the problem (for that my EntityManager wrapper parse the exception message).

However, note that exception codes differ from DB engines. In my case, I did it quick and used MySQL error codes, but managing different RDBMS implies more work.