feat(core): disambiguate string-based providers from class-based providers on error message

[micalevisk]

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/nestjs/nest/blob/master/CONTRIBUTING.md
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features) -- I'll update the docs as well if thi get merged

PR Type

What kind of change does this PR introduce?

• Bugfix
• Feature
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Build related changes
• CI related changes
• Other... Please describe:

What is the current behavior?

when some provider registered with a string token is missing, the error message is the same as the ones registered with classes

I believe this confuse people sometimes specially when the token is created by 3rd-party libs and, on the consumer side, there is a class with the same name

For example:

What is the new behavior?

I believe that this minor change would decrease the debugging time spent of resolving such errors. But I don't know if not having quotes around provider's name was intentional in first place.

Does this PR introduce a breaking change?

• Yes
• No(?)

[coveralls]

Pull Request Test Coverage Report for Build 037e170e-ef40-47ee-be06-a135c07aa6ba

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 92.639%

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Totals
Change from base Build fe06fc66-4e13-4e12-b2d5-e4752da0ecad:	0.0%
Covered Lines:	6469
Relevant Lines:	6983

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💛 - Coveralls

[kamilmysliwiec]

LGTM