Replace `write_error`-`return` with `raise`-`HTTPError` Syntax

[ric-evans]

remembering to return after writing the error is prone to create bugs, especially when the writing is nested in a call stack.

Instead, simply raise HTTPError. Some refactoring will be needed for validation/deconfliction methods that return bools.

[dsschult]

Example:

raise HTTPError(403, reason="authentication failed")