Improve warning and error messages for old routine.

[pp-mo]

This is a minimal improvement to the problems identified in #3449 with the error + warning messages produced by iris.analysis.calculus._construct_midpoint_coord.

Frankly there's a lot more that's wrong with this + I almost wish I hadn't bothered :

• the use of this private routine as a utility in the tests for another routine (_construct_delta_coord) is pretty horrible
• the optional use of the 'circular' keyword is only used by the test code, not the main iris codebase : So it could, and should be much simpler !
• the routine raises an error if called with circular=True when coord.circular does not exist, but it only emits a warning if coord.circular==False ((arrrrgh!))

[bjlittle]

omg 🤪

Some corners of iris are just distressing...