fix: don't pass invalid signal to subprocess.exit on crash

[flotwig]

• Closes Cypress crashes with misleading error message "Unknown signal: true" #15943

User facing changelog

Fixed an issue where crashes in Cypress would cause a misleading "Unknown signal: true" error after the actual crash message.

Additional details

• no users reported, but i have seen it a few times with random crashes

How has the user experience changed?

PR Tasks

• Have tests been added/updated?
• Has the original issue or this PR been tagged with a release in ZenHub?
• [na] Has a PR for user-facing changes been opened in cypress-documentation?
• [na] Have API changes been updated in the type definitions?
• [na] Have new configuration options been added to the cypress.schema.json?

[cypress-bot]

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[cypress]

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Test summary

9492 • 0 • 111 • 3 • 1

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Run details

Project	cypress
Status	Passed
Commit	2fe5556
Started	Apr 12, 2021 5:15 PM
Ended	Apr 12, 2021 5:27 PM
Duration	12:09 💡
OS	Linux Debian - 10.8
Browser	Multiple

View run in Cypress Dashboard ➡️

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Flakiness

	cypress/integration/retries.ui.spec.js		1
1	runner/cypress retries.ui.spec > opens attempt on each attempt failure for the screenshot, and closes after test passes

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[cypress-bot]

Released in 7.1.0.

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