Make a meaningful parser error when the EDI file loaded does not seem to be a valid EDI format

[cleftheris]

Because of the loose structure of the EDI format there is not definitive way to know the EDI file is not valid by checking for example the existence of the first tag that defines the transmission. Also this could limit the parsers ability to load partial structures/fragments.

Could potentially catch common human errors when someone accidentaly tries to load something that is in no way considered a valid EDI; for example check for other well known structures before starting to parse. For exapmple check the first non whitespace character usualy { for json and < for xml.

related to #158