fix: only send exception responses if header is not already sent

[wSedlacek]

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?

• 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: N/A

What is the new behavior?

When an exception is thrown it will trigger the exception filter to try to send a reply.
If a header was already sent then the exception filter will fail to send this reply.

Does this PR introduce a breaking change?

• Yes
• No

This change requires adapters to provide two new methods, isHeaderSent() and end()

Other information

[coveralls]

Pull Request Test Coverage Report for Build 76e79d94-3655-4331-aa6d-cef51bdcc711

• 6 of 8 (75.0%) changed or added relevant lines in 1 file are covered.
• 33 unchanged lines in 9 files lost coverage.
• Overall coverage decreased (-0.1%) to 94.053%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
packages/core/exceptions/base-exception-filter.ts	6	8	75.0%

Files with Coverage Reduction	New Missed Lines	%
packages/common/utils/is-uuid.ts	1	83.33%
packages/core/injector/module-token-factory.ts	1	94.29%
packages/core/router/sse-stream.ts	1	92.59%
packages/common/file-stream/streamable-file.ts	2	76.19%
packages/microservices/server/server-mqtt.ts	2	84.5%
packages/core/helpers/messages.ts	3	77.27%
packages/core/router/route-path-factory.ts	4	90.59%
packages/microservices/client/client-nats.ts	5	87.41%
packages/core/scanner.ts	14	89.45%

Totals
Change from base Build 2402d944-a9de-4ca1-82ac-e447b189c311:	-0.1%
Covered Lines:	5662
Relevant Lines:	6020

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💛 - Coveralls