This commit adds two error-handling cases to the function
attempt_dmr_backend. Specifically:
- In the case where the number of found GPUs is zero, we now
print a log message indicating this and return a NULL pointer
- In the case where we could not successfully create a backend
on any GPU, we now log a message indicating this and return
a NULL pointer
This allows us to provide more descriptive error messages,
as well as avoid a SEGFAULT (the function
`ensure_primary_backend_renderer_and_allocator` dereferences the pointer
given, which could be NULL until this patch) when these cases arise.
e22a386319