[Discussion] Textual error codes

This makes the error codes returned from the "transactional endpoint" of
the HTTP API be strings. String error codes has several benefits over
numeric error codes. Textual error codes are more descriptive and easier
to search for. Textual error codes also support both classification and
categorization, numerical error codes can only support either one or the
other.

Classification would be the distinction between transient and permanent
error conditions for example. Categorization would be the distinction
between different error sources (client side or server side), and
between different types of error (syntax errors, transaction errors,
et.c.).