fix: prevent UnhandledPromiseRejectionWarning when module resolution/parsing fails

[kherock]

This PR contains:

• bugfix
• feature
• refactor
• documentation
• other

Are tests included?

• yes
• no (see description)

Breaking Changes?

• yes (breaking changes will not be merged unless absolutely necessary)
• no

List any relevant issue numbers:

resolves #4217

Description

This fixes an instance where rollup can cause an unhandled rejection during normal programmatic usage. There isn't a simple way to test this since as of Node 15, a test would amount to ensuring that the process doesn't exit when you force a module resolution error.

I temporarily enabled @typescript-eslint/no-floating-promises with parserServices to check if there were any other places that had floating promises outside of ModuleLoader.ts and saw the following:

❯ npx eslint src --cache

/Users/kherock/Workspaces/rollup/src/ModuleLoader.ts
  341:3  error  Promises must be handled appropriately or explicitly marked as ignored with the `void` operator  @typescript-eslint/no-floating-promises

/Users/kherock/Workspaces/rollup/src/utils/queue.ts
  14:4  error  Promises must be handled appropriately or explicitly marked as ignored with the `void` operator  @typescript-eslint/no-floating-promises

/Users/kherock/Workspaces/rollup/src/watch/fileWatcher.ts
  19:3  error  Promises must be handled appropriately or explicitly marked as ignored with the `void` operator  @typescript-eslint/no-floating-promises
  21:4  error  Promises must be handled appropriately or explicitly marked as ignored with the `void` operator  @typescript-eslint/no-floating-promises
  30:4  error  Promises must be handled appropriately or explicitly marked as ignored with the `void` operator  @typescript-eslint/no-floating-promises

/Users/kherock/Workspaces/rollup/src/watch/watch-proxy.ts
  31:2  error  Promises must be handled appropriately or explicitly marked as ignored with the `void` operator  @typescript-eslint/no-floating-promises

/Users/kherock/Workspaces/rollup/src/watch/watch.ts
  97:4  error  Promises must be handled appropriately or explicitly marked as ignored with the `void` operator  @typescript-eslint/no-floating-promises

✖ 7 problems (7 errors, 0 warnings)

I didn't notice anything else that seemed dangerous, all the other cases were either already handled imperatively or were innocuous .close() calls that would only marginally benefit from a handler which logs a warning.

[codecov]

Codecov Report

Merging #4228 (ce1cd4d) into master (bce7624) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #4228   +/-   ##
=======================================
  Coverage   98.37%   98.37%           
=======================================
  Files         202      202           
  Lines        7260     7260           
  Branches     2119     2119           
=======================================
  Hits         7142     7142           
  Misses         58       58           
  Partials       60       60           

Impacted Files	Coverage Δ
src/ModuleLoader.ts	100.00% <100.00%> (ø)

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