See issue #62: sometimes our error are too terse.  Enrich them.  The
point here is first to look for a pattern that we could deploy
everywhere.

Before:

    In [2]: vcsn.Q.expression('a**').derivation('a')
    RuntimeError: q: star: invalid value: 1

    In [3]: vcsn.Q.expression('a**').constant_term()
    RuntimeError: q: star: invalid value: 1

    In [4]: vcsn.Q.expression('a**').derived_term('expansion')
    RuntimeError: q: star: invalid value: 1

    In [5]: vcsn.Q.expression('a**').derived_term('derivation')
    RuntimeError: q: star: invalid value: 1

After:

    In [2]: vcsn.Q.expression('a**').derivation('a')
    RuntimeError: q: star: invalid value: 1
      while computing derivative of: a**
                    with respect to: a

    In [3]: vcsn.Q.expression('a**').constant_term()
    RuntimeError: q: star: invalid value: 1
      while computing constant-term of: a**

    In [4]: vcsn.Q.expression('a**').derived_term('expansion')
    RuntimeError: q: star: invalid value: 1
      while computing expansion of: a**
      while computing derived-term of: a**

    In [5]: vcsn.Q.expression('a**').derived_term('derivation')
    RuntimeError: q: star: invalid value: 1
      while computing constant-term of: a**
      while computing derived-term of: a**

* vcsn/algos/constant-term.hh: Keep the expressionset, not
just the weightset.
Improve error messages.
Simplify tuple handling.
* vcsn/algos/derivation.hh: Improve error messages.
* vcsn/algos/derived-term.hh: Ditto.
(make_derived_term_automaton): Ditto.
* vcsn/algos/to-expansion.hh: Ditto.

* tests/bin/test.py (XFAIL): Also accept re patterns.
* tests/python/constant-term.py, tests/python/derivation.py,
* tests/python/expansion.py: Check error messages.