MessageHandlers are called by Sablotron to receive error reports,
display them, keep a log, trace etc.
- makeCode (severity, facility, code)
makes the "external" error code to report with log() or error()
call with facility = module id; severity = 1 iff critical.
code is the error code internal to Sablotron.
- log (code, level, fields)
pass code created by makeCode, level as necessary.
fields is a list of strings in form "field:contents".
distinguished fields include: msg, file, line, token
- error (code, level, fields)
for reporting errors, meaning as with log()