Use boxed str instead of String for error message
Use boxed str instead of String for error message

These are not modified while storing within the error and should be
faded out over time. Storing the Box<str> saves some space in the
representation with the size of a pointer. Note that DecodingError is
actually the largest struct right now due to it supporting some legacy
representations.

PistonDevelopers/image,PistonDevelopers/image