Use `error' where possible to signal errors.

Use `error' where possible to signal errors.
Remove (beep)s for warnings.
Change comment about the iif hook to reflect the actual reason.
Correct error in field numbering.

(forms-mode): Make `forms--elements' local before calling
`forms--process-format-list'.

(forms-save-buffer): `forms-write-file-filter' and
`forms-read-file-filter' were not called correctly.