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