Generate error message from readstat_error_t

[hadley]

It would be useful if there was a function that turned the error code into a string for reporting messages to the user.

[hadley]

Oops, I guess that's readstat_error_message()!

[hadley]

Although it's a little weird that the parser functions return int, but readstat_error_message() needs a readstat_error_t

[hadley]

Also readstat_error_message doesn't appear to be defined anywhere

[evanmiller]

Oops, forgot to add readstat_error.c to the repository. Added:

5b2271b

The parser functions should return readstat_error_t. I'll create a separate issue.