Improve the error message when pcolormeshing a non-bounded cube

[pelson]

qplt.pcolormesh(iris.load(iris.sample_data_path('uk_hires.pp'))[0][0, 0, ...])

Generates the exception:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "lib/iris/quickplot.py", line 158, in pcolormesh
    result = iplt.pcolormesh(cube, *args, **kwargs)
  File "lib/iris/palette.py", line 138, in wrapper_func
    return func(*args, **kwargs)
  File "lib/iris/plot.py", line 705, in pcolormesh
    return _draw_2d_from_bounds('pcolormesh', cube, *args, **kwargs)
  File "lib/iris/plot.py", line 214, in _draw_2d_from_bounds
    result = _map_common(draw_method_name, None, iris.coords.BOUND_MODE, cube, data, *args, **kwargs)
  File "lib/iris/plot.py", line 349, in _map_common
    lats, lons = iris.analysis.cartography.get_lat_lon_contiguous_bounded_grids(cube)
  File "lib/iris/analysis/cartography.py", line 196, in get_lat_lon_contiguous_bounded_grids
    lons = lon_coord.contiguous_bounds()
  File "lib/iris/coords.py", line 548, in contiguous_bounds
    self._sanity_check_contiguous()
  File "lib/iris/coords.py", line 533, in _sanity_check_contiguous

[pelson]

I should add, the easy fix is to add bounds to the appropriate coordinates (in this case grid_latitude and grid_longitude)

[rhattersley]

With the change we now get:

ValueError: Invalid operation for 'grid_longitude', with 0 bounds. Contiguous bounds are only defined for coordinates with 2 bounds.

It's a lot better than the original, but still obscure for the end-user. Perhaps a job for exception translation? i.e. The plot routine catches the exception and re-raises as something intelligible (which could still just be a simple ValueError).

[pelson]

Note: Being addressed in #172.

[esc24]

Fixed by #204 and #172