Standardize format for library error messages

[ericnordelo]

Currently, we have some small inconsistencies like Account: no reentrant call and ReentrancyGuard: reentrant call, that is, for example, an inconsistency between describing the expected behavior vs describing the issue.

I think we should always use the describing the issue version (ReentrancyGuard: reentrant call in the example), besides following the "{namspace}: {issue}" format and always in the present tense.

We can add an entry to the CONTRIBUTING guide for this, what are your thoughts @martriay @andrew-fleming?

[andrew-fleming]

No arguments from me—I agree with using the describing the issue approach and keeping the error messages consistent. If this is accepted, I'd suggest that we be mindful to try and include these changes before the next breaking-changes release

[martriay]

Superseded by #696