Add error handling to backend creation

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.

0 files changed, 0 insertions, 0 deletions