Improve misleading error message in check_for_nans

[billsacks]

The check_for_nans routine is used to check import and export fields to / from CTSM for NaN values. However, even though this same function is used for both imports and exports, the error message is always, "One or more of the output from CLM to the coupler are NaN".

We should add an argument to this subroutine saying whether it's being used for import or export and adjust the text in the error message accordingly, so that it either reads "One or more of the outputs from CTSM to the coupler are NaN" or "One or more of the inputs from the coupler to CTSM are NaN".