Consistent handling of error messages

We should clean up and make uniform the way we print error messages.

My suggestion is the following "canonical" error message:

Unable to <task>. Reason: <reason>.

A new function in log.h has been added to handle this in the case of a simple value comparison:

check_value(value, valid_value, task, value_name)

Example outpout:

*** Error: Unable to create boundary condition. Reason: Illegal value 1 for value rank, should be 0.

We should also add more extensive checks of input arguments and give as helpful error messages as we possibly can.