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

Nobody has claimed this yet.

This was originally [reported in OpenTitan](https://github.com/lowRISC/opentitan/issues/4689) 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?"

Start by tracing how the --tool value expands import_cfgs entries such as common_project_cfg.hjson and {tool}.hjson. The work is done when a missing tool-specific file produces a helpful error that identifies the missing path and the wildcard values or otherwise clearly explains the invalid --tool value.

Written by the indexing model from the issue text.