Duplicate implementation of error-to-string functions

There are duplicate implementations of error-to-string functions in the following places:

• lib_dec.c: IVAS_DEC_GetErrorMessage
• lib_enc.c: IVAS_ENC_GetErrorMessage
• ivas_error.h: ivas_error_to_string

Best way to clean this up while avoiding too many changes:

1. Move all error-to-string mapping to ivas_error_to_string
2. Call ivas_error_to_string internally within IVAS_[ENC|DEC]_GetErrorMessage