fix(microservice): handle promise return by 'registerModules' properly

[danielscw]

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)

PR Type

What kind of change does this PR introduce?

[x] 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?

Issue Number: #5135

What is the new behavior?

Allow error inside onModuleInit to properly bubble up and be able to be catch inside bootstrap function.
Resolve the UnhandledPromiseRejectionWarning issue.

Does this PR introduce a breaking change?

[ ] Yes
[x] No

Other information

[coveralls]

Pull Request Test Coverage Report for Build 4f36817b-7262-4880-a347-4761fa238994

• 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 94.817%

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Totals
Change from base Build ffdcbe90-9390-4ce6-ac40-f441be5ae96a:	0.0%
Covered Lines:	4921
Relevant Lines:	5190

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💛 - Coveralls

[kamilmysliwiec]

Thanks!