Convert a return code integer to a text string. This string can be
used to output a diagnostic message to the user.
PUNYCODE_SUCCESS
Successful operation. This value is guaranteed
to always be zero, the remaining ones are only guaranteed to hold
non-zero values, for logical comparison purposes.
PUNYCODE_BAD_INPUT
Input is invalid.
PUNYCODE_BIG_OUTPUT
Output would exceed the space provided.
PUNYCODE_OVERFLOW
Input needs wider integers to process.
RETURN VALUE
Returns a pointer to a statically allocated string
containing a description of the error with the return code rc.
