Feature request: Better output/error feedback

[TechnikTobi]

May I suggest that with a released version there should be no stderr output either an appropriate return Result or some function that allows the caller to read the error cause in case the read result (i.e. metadata) length is Zero, meaning that LittleExif did not find acceptable exif information.

Originally posted by @mschnell1 in #37 (comment)

[mschnell1]

Supposedly using a Result as return is not a good idea, as the user would like to use the new created Metadata struct (and hiding same in an Error type seems rather weird). Hence a function that returns the latest reason why metadata could not be decoded might be the most viable option.