User friendly config parse errors in tab --check

[jdisanti]

Thanks for building this!

I found a config parsing error that could be improved, but it looks fairly involved. Using the following config:

workspace:
  - repo: some-repo/
  - tab: some-repo-1
  - doc: "some top-level tab"

Will result in an error that requires cross comparing with an example or looking at an enum in code to figure out why it didn't work (the doc key shouldn't have its own hyphen in this particular example):

$ tab --check
1 error was detected:
    - Failed to deserialize config at path: /home/jdisanti/src/tab.yml - error: data did not match any variant of untagged enum Config

Ideally, this error message would be more along the lines of:

1 error was detected:
    - /home/jdisanti/src/tab.yml line 4, col 3: unknown workspace field 'doc'

This appears to be a serde bug: serde-rs/serde#773

I think there are two options for improving the error:

1. Refactor config deserialization to work around the serde issue
2. Contribute a fix to serde