[llvm-readobj] - Refactor the MipsGOTParser<ELFT> to stop using report_fatal_error().

[llvm-readobj] - Refactor the MipsGOTParser<ELFT> to stop using report_fatal_error().

`MipsGOTParser` is a helper class that is used to dump MIPS GOT and PLT.
There is a problem with it: it might call report_fatal_error() on invalid input.
When this happens, the tool reports a crash:

```
# command stderr:
LLVM ERROR: Cannot find PLTGOT dynamic table tag.
PLEASE submit a bug report to https://bugs.llvm.org/ and include the crash backt
race.
Stack dump:
...
```

Such error were not tested. In this patch I've refactored `MipsGOTParser`:

I've splitted handling of GOT and PLT to separate methods. This allows to propagate
any possible errors to caller and should allow to dump the PLT when something is wrong
with the GOT and vise versa in the future.

I've added tests for each `report_fatal_error()`
and now calling the `reportError` instead. In the future we might want to switch to
reporting warnings, but it requres the additional testing and should
be performed independently.

I've kept `unwrapOrError` calls untouched for now as I'd like to focus on eliminating
`report_fatal_error` calls in this patch only.

Differential revision: https://reviews.llvm.org/D83225