Fix PR117. The error message is "keywords must be strings". Perhaps

Fix PR117.  The error message is "keywords must be strings".  Perhaps
not as descriptive as what Barry suggests, but this also catches the
(in my opinion important) case where some other C code besides apply()
constructs a kwdict that doesn't have the right format.  All the other
possibilities of getting it wrong (non-dict, wrong keywords etc) are
already caught so this makes sense to check here.