Validate upload token before sending "everything" to the server via codecov CLI

[vlad-ko]

What product do you want to improve?
Codecov CLI

Is your feature request related to a problem? Please describe.
I tired to setup the codecov cli to handle the report uploads. Unfortunately the token I had stored in Git secrets was either malformed or otherwise invalid. This was impossible to figure out, without going to the codecov logs. A customer wouldn't be able to decipher this on their own.

Describe the solution you'd like
Validate the token for UUID schema before sending "everything" to the server.

Describe alternatives you've considered
No other solutions were considered.

Additional context
Detailed recap.
I tried to pass in the token to the codecov cli, as described in the github docs. The output of the CLI in the CI showed that "some token" was available to proceed with the upload:

Run 
codecov --verbose create-commit --fail-on-error 
codecov --verbose create-commit --fail-on-error
  codecov --verbose create-report --fail-on-error
  codecov --verbose do-upload --fail-on-error --flag service --name service-tests
  shell: /usr/bin/bash -e {0}

  env:

    COMPOSER_PROCESS_TIMEOUT: 0
    COMPOSER_NO_INTERACTION: 1
    COMPOSER_NO_AUDIT: 1
    CODECOV_TOKEN: ***

I interpreted the above output as though the token was accepted and parsed by the CLI.

However during upload we saw a 500 error, that is pretty generic. The only way to figure out what was caused by the malformed token was to look at the GCP logs. A customer would likely open a support ticket at this point.

A bonus here would be to return a message from the server that the 500 was caused by the "bad token".