[ fix #3783 ] Improve error message for invalid C FFI declarations

[cypheon]

Description

As reported in #3783, the error message you receive when omitting the library name from a C FFI declaration is confusing and obscures the root cause.
Since the current implementation of Chez Scheme FFI into C depends on the use of a dynamic library, the error message should be explicit about it.

TODO

• decide on exact wording of the error message
• check behaviour on Racket and RefC backends
• add tests

Self-check

• If this is a fix, user-facing change, a compiler change, or a new paper
  implementation, I have updated CHANGELOG_NEXT.md
• I confirm that this contribution did not involve GenerativeAI nor Large Language Models.