Add more graceful error handling when a direct compiler command is used

[matt-dees]

build-bom silently refuses to add .llvm_bitcode section to the build output when a direct compiler command like

build-bom generate-bitcode --clang clang-9 -- clang-9 test.c -c

is used instead of a build system call. It would be nice to notify the user of the problem.

[kquick]

Fixed by #71