More idiomatic error classes in Python

[savi2w]

Complementing #215 Putting this here so it doesn't get lost

Having nested class for each error like this is pretty un-idiomatic in python. I'm not sure how to expose our errors in a better way though

_Originally posted by @rfk in https://github.com/mozilla/uniffi-rs/pull/215#discussion_r467660744_

┆Issue is synchronized with this Jira Task ┆Issue Number: UNIFFI-13

[rfk]

One thought here is, it would be nice to make the container class a superclass for the specific cases, so we could have something like:

try:
    a_function_that_throws():
except ArithmeticError.IntegerOverflow:
    do_something()
except ArithmeticError.DivideByZero:
    do_something_else()

But could also generically catch any ArithmeticError and then inspect it in more detail, like:

try:
    a_function_that_throws():
except ArithmeticError as e:
    do_something()
    if isinstance(e, ArithmeticError.IntegerOverflow):
        so_something_extra()

Idiomatic python would have these all in one flat namesace (e.g. ArithmeticError, IntegerOverflowError and DivideByZeroError classes all in the top level of the module) but being able to do the above might be a nice compromise.