feat(core): handle errors in observable streams

[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?

Previously when an error was thrown in an observable as part of an SSE request the server would crash.

Issue Number: #8730

What is the new behavior?

Now errors are properly handled within SSE request so that if an error occurs then an error message event is sent and the connection is closed.

Does this PR introduce a breaking change?

• [] Yes
• No

Breaking changes have been removed from scope

Other information

[wSedlacek]

@jmcdo29 I tried the approach of sharing logic between REST and SSE request and this is what I ended up with.
I had to adapt several functions to return Observables instead of promises so that the logic could be shared at the proxy level.

This had the side effect of sharing the behavior of unsubscribing when a request is closed from REST handlers as well.

Let me know if you like this direction and I can work on adding some test and docs.

[coveralls]

Pull Request Test Coverage Report for Build 65ceea40-0985-4f1e-b40c-0c581c2903af

• 14 of 16 (87.5%) changed or added relevant lines in 2 files are covered.
• 33 unchanged lines in 9 files lost coverage.
• Overall coverage decreased (-0.1%) to 94.053%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
packages/core/router/router-response-controller.ts	12	14	85.71%

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

[kamilmysliwiec]

LGTM

[soyuka]

we should definitely forward the error and let the user do this, this doesn't allow us to do so anymore.