Exception handler tests

[Anish9901]

Related to #3524

This adds tests for the error handling decorator handle_rpc_exceptions, and verifies a function wrapped always raises a RPCException

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[mathemancer]

Test looks good, but the imports are (re)ordered strangely.

Is there a reason for the non-standard import sorting? If not, just use the style I recommended, please.

[mathemancer]

Did an automated tool reorder these imports? The new order is pretty non-standard. I'd rather have:

from http.client import CannotSendRequest

from django.core.exceptions import FieldDoesNotExist
from psycopg.errors import BadCopyFileFormat
import pytest
from sqlalchemy.exc import IntegrityError

from db.functions.exceptions import UnknownDBFunctionID
import mathesar.rpc.exceptions.error_codes as err_codes
from mathesar.utils.connections import BadInstallationTarget

[mathemancer]

The imports should be grouped into:

1. standard library
2. 3rd party libraries
3. local libraries (our code)

Sorting should be lexicographical by non-keyword within those groups.

[mathemancer]

These are also sorted strangely.

[mathemancer]

After discussing with @Anish9901 , I'm convinced there is a logic to the sorting of the imports in these test files.

@Anish9901 , Please add a comment or two about that logic, and then go ahead and merge this.

[Anish9901]

I've sorted & grouped the imports based on "what's being tested" rather than "where are we importing from". Furthermore, if we try to see the imports through the lens of Arrange, Act and Assert we'll notice that the following imports are required for the Arrange step for the tests defined:

from modernrpc.exceptions import RPCException
from psycopg.errors import BadCopyFileFormat
from django.core.exceptions import FieldDoesNotExist
from mathesar.utils.connections import BadInstallationTarget
from db.functions.exceptions import UnknownDBFunctionID
from http.client import CannotSendRequest

Similarly, we can think of the following import as something which will be Acted upon or rather the function that is to be tested.

from mathesar.rpc.exceptions.handlers import handle_rpc_exceptions

The first line is reserved for the imports that would be imported multiple times throughout our test files and for any imports that are required for rudementry python/pytest functions e.g. os, pytest etc.