derivations: Raise an error when a module file is not found.

derivations: Raise an error when a module file is not found.

Suggested by Jookia.

* guix/derivations.scm (&file-search-error): New error condition.
(search-path*): Raise it when 'search-path' returns #f.
* guix/gexp.scm (search-path*): Remove.
* guix/ui.scm (call-with-error-handling): Add case for
'file-search-error?'.
* tests/derivations.scm ("build-expression->derivation and invalid
module name"): New test.