Cleanup parameter/return type checking code

a) Extract all the logic for error message formatting into a separate
function that is reused for everything.
b) Extract the logic for whether a value matches a type into two
functions, one for user types, one for internal ones.

This tries to fight the increasing code duplication in the type
checking code, whether everything was repeated at least four times
and parts of the error message handling were repeated in dozens of
branches.

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