tools/ocaml: abi-check: Improve output and error messages

tools/ocaml: abi-check: Improve output and error messages

In the generated C, add some comments saying where we found the ocaml
type.  This helps with debugging.  (I considered emitting #line
directives but decided this would be more confusing than helpful.)

Improve two dies.

Use better-named filehandles (perl prints thier names when it dies).

Signed-off-by: Ian Jackson <ian.jackson@eu.citrix.com>
Acked-by: Andrew Cooper <Andrew.Cooper3@citrix.com>
Acked-by: Christian Lindig <christian.lindig@citrix.com>