All errors in functions in the (lepton config) are reported using
one of two keys:
system-error key.
config-error key.
When a config-error is signalled, data part of the error
arguments is a list containing one of the following symbols:
unknown-encodingThe text being parsed was in an unknown encoding.
parseThe configuration data wass ill-formed.
key-not-foundA requested configuration key was not found.
group-not-foundA requested configuration group was not found.
invalid-valueA configuration value could not be parsed into the requested format.