[Support] Make DataExtractor error messages more clear

Summary:
This is a result of the discussion at D78113. Previously we would be
only giving the current offset at which the error was detected. However,
this was phrased somewhat ambiguously (as it could also mean that end of
data was at that offset). The new error message includes the current
offset as well as the extent of the data being read.

I've changed a couple of file-level static functions into private member
functions in order to avoid passing a bunch of new arguments everywhere.

Reviewers: dblaikie, jhenderson

Subscribers: hiraditya, MaskRay, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D78558