EASY: better error messages

[m4b]

I.e., this error message sucks:

type is too big (68719476740) for 7828

I just realized tho, that we can "recover" the error; if we simplify our internal types to return TooBig, we can then have pread "catch" TooBig errors, and then recompute the offset, requested size, the offset requested at, and size of the bytes for us, all in a nice package