More informative error messages

[jacobpennington]

Often with the current NEMS, errors are difficult to debug because they're primarily raised by dependencies and may have little information relevant to NEMS (beyond hunting through the traceback for NEMS code.

Instead, core nems functions/classes should do some reasonable checks & raise their own errors where appropriate. This isn't necessary everywhere: for example, checking if a key exists in Class.special_dict is unnecessary since the IndexError that a missing key would raise is already straightforward (i.e. the built-in error already gives you the releveant info - missing key - and it's directly tied to that dictionary, rather than separated by 4 levels of other packages). But in cases where a malformed input, missing key, etc. would result in an obtuse error a "long way" down the line, NEMS code should raise its own error.