MAINT: improve C accum error handling

[tylerjereddy]

• returning -1 from a public API C function isn't sufficient to provide useful error information when working in Python/CFFI--it only tells you something went wrong if you check for a return code in the first place (which isn't normally done in Python anyway--normally you end execution at the error point and exit() with an appropriate error message)

• when there are multiple conditions that can trigger the -1 return value, the situation is even worse, one literally has to printf sprinkle the source to figure out what went wrong where

• as a compromise, I'll leave the -1 approach in since that is quite common in standard C, but I'm going to add in prints to stderr so that Python can then intercept the -1 and refer the user to stderr

• also, darshan_accumulator_inject() assumed that the module_id was reasonable because it was set/checked in
  darshan_accumulator_create(), however my experience in ENH: Python derived/accum interface #839 was that the accumulator memory location can get freed, or not properly set at CFFI boundary after calling the creation function, so I think the assumption that a previous function was called and worked perfectly is too fragile--I'm adding error handling to prevent a hard segfault on nonsense values of that structure member as a result

[tylerjereddy]

Unique return codes for different situations would likely be even more useful--i.e., preventing a segfault due to corruption vs. "this module doesn't support accum interface" or whatever.

[carns]

Thanks @tylerjereddy . I'm happy with the darshan-logutils-accumulator.c improvements (adding stderr error msgs is a particularly good idea), but the .py code isn't correct (see #839 for issues with the fn prototype in particular). I'm not sure how to proceed exactly, but I think any of the following would be fine, and I can help with the 3rd option if that's preferred.

• make a new PR with just the .c changes
• wait for resolution of the bindings/prototype issues in ENH: Python derived/accum interface #839 and then apply the same strategy here
• add a munit C code unit test for the error handling (this can be tested in C)

[shanedsnyder]

I agree the changes in the C library here are definitely helpful.

I think I'd vote for us to fix the issues in #839 first, and then rebase on that and adopt the same style code for the test in this PR just for consistency.