[GH-4553] Simplify error handling in the OCI8 driver

[morozov]

Q	A
Type	improvement
BC Break	no

See #4553.

While reviewing #4581, I had an impression that the oci8 extension only reports fetching errors via the PHP error mechanism and doesn't set the error status available via oci_error(). I did some debugging of the extension and it turned out the error handling works both ways: via the PHP errors and the error status on the statement.

This patch simplifies the original approach:

1. No longer need to set and reset the error handler.
2. No longer need to parse error messages.
3. No additional exception class.
4. I didn't check that yet but the resulting exception message should be closer to the one that PDO would produce.

[morozov]

@amenning I did some digging into the extension and simplified your patch a bit. Could you take a look?

[amenning]

This is a great improvement. Code looks good and I see no issues. This covers the original example and will be far better for properly catching and reporting a wider set of related errors.

[greg0ire]

Nice simplification !