Improve error message from Field.set_construct when domain axes do not exist

[davidhassell]

Improve error message from Field.set_construct when domain axes do not exist.
For example:

>>> z = cf.Field()
>>> nt = 450
>>> climdim = cf.DimensionCoordinate(data=cf.Data(numpy.arange(nt)))
>>> z.set_construct(climdim)
<snip>
ValueError: Can't insert  DimensionCoordinate: Ambiguous shape: (450,). Consider setting the 'axes' parameter.

It would be better in this case to indicate that a domain axis needs to be created first.

[davidhassell]

When fixed, the error message in this case would say

ValueError: Can't insert <CF DimensionCoordinate: (450) >: There is no domain axis construct with size 450.

@JonathanGregory - is that OK?

[JonathanGregory]

Perfect, thanks