Currently to check if an expression is invalid, we compute its
constant-term and catch errors: indeed `1*` will fail.

Unfortunately constant-term can also fail because of operators
it does not support, such as @ and {\}.  We must tell the difference.

See issue #60.

* vcsn/algos/constant-term.hh: Report errors for compose too.
* vcsn/algos/is-valid-expression.hh: Improve error messages.

* tests/bin/test.py: Improve error messages.
* tests/python/constant-term.py:
* tests/python/is-valid.py: Prefer q to r.
Check more cases.