[Feature]: Improve error messages when attempting retrieve coords/coord bounds for an axis

[tomvothecoder]

Is your feature request related to a problem?

In PR #246, a new method called _get_coord_var() was added, which checks if the supplied axis key can be interpreted using the axis attribute:

xcdat/xcdat/axis.py

Lines 327 to 355 in 350ac8b

	def _get_coord_var(dataset: xr.Dataset, axis: GenericAxis):
	"""Gets a coordinate variable using its "axis" attribute.
	Parameters
	----------
	dataset : xr.Dataset
	The dataset.
	axis : GenericAxis
	The axis.
	Returns
	-------
	xr.DataArray
	The coordinate variable.
	Raises
	------
	KeyError
	If the coordinate variable was not found in the dataset.
	"""
	try:
	coord_var = dataset.cf[axis]
	except KeyError:
	raise KeyError(
	f"A '{axis}' coordinate variable was found in the dataset. Make sure "
	"the coordinate variable exists in the Dataset, and its 'axis' attribute "
	f"is set to '{axis}'."
	)
	return coord_var

However, when attempting to retrieve coord bounds, we raise an ambiguous error message if they aren't found:

xcdat/xcdat/bounds.py

Lines 179 to 183 in 350ac8b

	try:
	axis = GENERIC_AXIS_MAP[axis]
	bounds = self._dataset.cf.get_bounds(axis)
	except KeyError:
	raise KeyError(f"{axis} bounds were not found, they must be added.")

Describe the solution you'd like

There are several reasons for why coord bounds can't be retrieved (#164 (comment)):

I plan on opening a separate PR to improve error messages when attempting to fetch coordinate variables or bounds data variables using cf_xarray and CF-compliant "axis" and/or "standard_name" attributes.

This KeyError is possibly being raised because:

1. There is no Z coordinate variable
2. There is a Z coordinate variable, but the "axis" attribute is not set to "Z"
3. There is a Z coordinate variable and the "axis" attribute is set to "Z", but the "bounds" attribute is not set to the name of the bounds data variable
4. The bounds data var doesn't exist and need to be added.

We need a clearer error message based on which case is true.

Describe alternatives you've considered

No response

Additional context

No response

[durack1]

@tomvothecoder FYI, the upcoming Python 3.11 release (October 2022) plans to include "Fine-Grained Error Locations in Tracebacks" which looks like a great improvement over 3.10 and earlier. The Exception Groups also looks like a great optimization.

Might be useful to ponder as you're working on this feature

See https://www.python.org/downloads/release/python-3110b3/