Omit numbering of errors if there's only one

[ivan-aksamentov]

Currently the numbering of the list of errors is shown even if there is only one error:

Error:
   0: There is only one error

Location:
...

I propose to remove the numbering in this case, because some of our users are confused by this 0: somehow. Some think that it's an erroneous file name or even line number (i.e. if a file parsing error is reported) or an error code.

The numbering of the error messages is not something that's common in other CLI frameworks.

[ten3roberts]

Would you also like if we made the error numbering configurable too, such that it can be entirely removed or replaced with a dash-style list?

- Error message
- Other error message

[monkeydom]

+1 here. I'd like to use color-eyre in user facing tools. However, the great dev features don't provide for a good user experience where I would like it to just show the first error and all sections and suggestions. Happy to whip up a patch here if it is appreciated.