Enable warning for the `clippy::question_mark` lint

[Kanabenki]

Objective

• Enable the warn level for the clippy::question_mark lint. It seems rather uncontroversial and general good practice to me.

Solution

• Enable the lint.
• Use the question mark/ok adapter to simplify some Option/Result returns. I can revert that and keep only the changes highlighted by the lint if preferable.
• Fix some trailing whitespace.

[Kanabenki]

There are two other question mark related lints that could be added alongside this one, which currently do not trigger anywhere in the codebase (unless we want to go with always one lint per PR):

• needless_question_mark
• needless_return_with_question_mark

[alice-i-cecile]

One lint per PR is nice: it lets us evaluate the impact and discuss them seperately.

[alice-i-cecile]

@mockersf do you still want to mark this PR as controversial?