Re: comint read-only prompt

[Richard Stallman]

    Given how often this sort of complaint turns up (that a read-only property 
is
    a good solution except for the error message), how about this idea: If a
    signalling an error due to a read-only property, if the property's value is 
a
    string, use that as the error message rather than the default.

Some such festure seems like a good idea.
The basic handling of error message strings in Emacs is
rather inflexible, so there is no existing feature we
can use to do this.  Doing it with a special-case kludge
might be the best way, since it is easy.