make(1): in lint mode, exit with error status on errors

Calling Parse_Error during parsing has always led to a nonzero exit
status.  Calling Parse_Error later, when expanding the shell commands,
has had no effect on the exit status.  Neither had calling Error.

To make make a reliable tool, it has to report errors as they occur.
Enable this strict behavior in lint mode for now.  Lint mode has to be
enabled explicitly, preserving the default behavior.

(rillig)