* Added "source" argument to lg_add_pattern_list to indicate the source
of the patterns (e.g., a filename or path). This allows us to report
errors more naturally when multiple consecutive calls to
lg_add_pattern_list are made before handling any errors.

* Added Pattern, EncodingChain, and Source members to LG_Error. This
should make it possible for the consumer of LG_Errors to produce better
error messages.