Improve error messages for invalid record definitions.

Improve error messages for invalid record definitions.

* module/srfi/srfi-9.scm (%define-record-type): Accept additional 'form'
  parameter which contains the original form of 'define-record-type' or
  'define-immutable-record-type'.  Add elaborate pattern guard which
  raises descriptive syntax errors for specific errors, and a fallback
  pattern to catch anything else.
  (define-record-type): Pass 'form' parameter to %define-record-type.

* module/srfi/srfi-9/gnu.scm (define-immutable-record-type): Pass 'form'
  parameter to %define-record-type.

* test-suite/tests/srfi-9.test: Add tests.