Give a better error message on a silly --tool value #233

This was originally reported in OpenTitan in 2020.

The basic problem is that dvsim tries too hard to load tool-specific files. For example:

I think it's kind of nice that dvsim doesn't care what tools you have installed, but surely we can spit out a more helpful error message. That file is getting loaded because OpenTitan has this in common_sim_cfg.hjson:

  import_cfgs:      ["{proj_root}/hw/data/common_project_cfg.hjson",
                     "{dv_root}/tools/dvsim/common_modes.hjson",
                     "{dv_root}/tools/dvsim/fusesoc.hjson",
                     "{dv_root}/tools/dvsim/bazel.hjson",
                     "{dv_root}/tools/dvsim/{tool}.hjson"]

I wonder whether we should add a magic "test_files" key or similar that would allow the tool die with a message saying "The file path/to/foobar.hjson does not exist. The path was computed from the following wildcards: is one of them wrong?"