[libomptarget][amdgpu] Remove majority of fatal errors

[libomptarget][amdgpu] Remove majority of fatal errors

Replaces most calls to exit() with returning an error to the library entry
point. Minor changes to error handling for clear bugs, remove some dead code.

Each exit() call site replaced is either in a library entry point or a
function that already returns error codes on some paths. The existing handling
is not well tested but replacing exit() with a fallback path should be a strict
improvement.

Remaining two early exit points are an abort() from a callback and exit() from
within msgpack. Fixes for those are less obvious and left for a later patch.

Reviewed By: pdhaliwal

Differential Revision: https://reviews.llvm.org/D102346