Improve error messages for non-matching `elif`/`else` statements

[thatbirdguythatuknownot]

BPO	45759
Nosy	@aroberge, @lysnikolaou, @pablogsal, @AlexWaygood, @thatbirdguythatuknownot
PRs	bpo-45759: Better error messages for non-matching 'elif'/'else' state… #29481 bpo-45759: Better error messages for non-matching 'elif'/'else' statements #29513 bpo-45759: Re: Better error messages for non-matching 'elif'/'else' statements #29775

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GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2021-11-08.23:51:38.287>
labels = ['interpreter-core', 'type-bug', '3.10', '3.11']
title = 'Improve error messages for non-matching `elif`/`else` statements'
updated_at = <Date 2021-11-25.12:15:15.681>
user = 'https://github.com/thatbirdguythatuknownot'

bugs.python.org fields:

activity = <Date 2021-11-25.12:15:15.681>
actor = 'Crowthebird'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Parser']
creation = <Date 2021-11-08.23:51:38.287>
creator = 'Crowthebird'
dependencies = []
files = []
hgrepos = []
issue_num = 45759
keywords = ['patch']
message_count = 2.0
messages = ['405990', '405997']
nosy_count = 5.0
nosy_names = ['aroberge', 'lys.nikolaou', 'pablogsal', 'AlexWaygood', 'Crowthebird']
pr_nums = ['29481', '29513', '29775']
priority = 'normal'
resolution = None
stage = 'patch review'
status = 'open'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue45759'
versions = ['Python 3.10', 'Python 3.11']

[thatbirdguythatuknownot]

While fixing errors in a program, I encountered this:

if a == b + 2:
    c = sqrt(b) + a**2
    elif a == b + 3:
    ^^^^
SyntaxError: Invalid syntax

It should be giving an IndentationError or a better error message at least.

[thatbirdguythatuknownot]

I'd like to expand this to more than just elif/else statements inside if` statement blocks and make an error for other cases as well. I currently have a PR that has a general error to accommodate those cases.

[iritkatriel]

CC @pablogsal .