gapit benchmark should not require -dumptrace to run.

[hevrard]

gapit benchmark --api vulkan de.saschawillems.vulkanBloom
[...]
Error: stat benchmark.gfxtrace: no such file or directory

This error does is not very helpful. It looks like the -dumptrace argument is required for benchmark, the error message should say so.

[AWoloszyn]

The -dumptrace should not be required, so we should fix that part of it. (Just fix the case where it is not provided)