Improve error message when given invalid hosting configuration.

[taeold]

Given invalid hosting configuration, Firebase CLI fails in an unhelpful way.

e.g.

{
  "hosting": {
    "public": "public",
    "rewrites": [
      {
        // missing "destination", "function", or "run"
        "source": "**"
      }
    ]
  }
}

Before:

$ firebase deploy --only hosting
...
Error: An unexpected error has occurred.

$ firebase deploy --only hosting --debug
...
Error: Never has a value ([object Object]). This should be impossible

After

$ firebase deploy --only hosting
...
Error: Invalid hosting rewrite config in firebase.json. A rewrite config must specify 'destination', 'function', 'dynamicLinks', or 'run'

Fixes https://groups.google.com/a/google.com/g/firebase-discuss/c/VoQ0XkNnZFs

[bkendall]

Is this the only list of situations where this can happen? If so, then this should bef ine

[taeold]

Yes!

[codecov-commenter]

Codecov Report

Base: 56.13% // Head: 56.13% // Increases project coverage by +0.00% 🎉

Coverage data is based on head (220cbfc) compared to base (d6826ee).
Patch coverage: 0.00% of modified lines in pull request are covered.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #5533   +/-   ##
=======================================
  Coverage   56.13%   56.13%           
=======================================
  Files         317      317           
  Lines       21503    21505    +2     
  Branches     4388     4388           
=======================================
+ Hits        12070    12072    +2     
- Misses       8370     8371    +1     
+ Partials     1063     1062    -1     

Impacted Files	Coverage Δ
src/deploy/hosting/convertConfig.ts	72.41% <0.00%> (-1.28%)	⬇️
src/functional.ts	91.48% <0.00%> (ø)
src/emulator/auth/state.ts	85.43% <0.00%> (+0.56%)	⬆️

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