src/module-{proto,impl}.lisp, etc.: Don't output erroneous modules.

src/module-{proto,impl}.lisp, etc.: Don't output erroneous modules.

Add a new reader `module-errors' which reports the number of errors
found while processing the module.  Notice (but decline to handle)
errors while loading modules.  And, in the front-end, don't try to
output erroneous modules.

This fixes an annoying problem where errors are reported and cause
`make' to fail, and then a subsequent `make' apparently succeeds,
possibly with bogus C code.