improvement for error message.

[Chris Dams]

Hello,

In the function ex::subs(const ex & e, unsigned options) if the first
argument is neither a relation_equal nor a list, the error message
"basic::subs(ex): argument must be a list" gets thrown. I think it would
be a good idea to change this into "basic::subs(ex): argument must be a
relation_equal or list". Imagine me trying to type a relation_equal,
getting some parentheses wrong and ending up with something that is not a
relation_equal nor a list. In that case the former error message makes me
think "when did it happen that I cannot use a relation anymore as the
argument of a substitution". The latter error message might put me on the
right track of starting to wonder why the thing I thought was a relation
turns out not to be.

Bye,
Chris Dams