Clarify ValueError for plot_profile()

[ewquon]

Calling plot_profile() with specified times=..., for which any of the specified times does not exist throws a cryptic message:

ValueError: x and y must have same first dimension, but have shapes (0, 200) and (200,)

where 200 is the number of height levels. This backtraces to https://github.com/a2e-mmc/mmctools/blob/master/plotting.py#L819.

This should instead say something along the lines of "The requested time does not exist in the dataset" and then skip that particular profile.