[runner][error] Avoid nm error message on non-lib files
[runner][error] Avoid nm error message on non-lib files

Re-fixes #251, this time by capturing the stderr from `nm` instead of
replacing it.

cgreen-devs/cgreen,cgreen-devs/cgreen,cgreen-devs/cgreen,cgreen-devs/cgreen,cgreen-devs/cgreen