cluster: show message for impossible fix

[hhkbp2]

What problem does this PR solve?

This PR fixes issue #335

What is changed and how it works?

Add message for failed checking result that's impossible to fix by tiup cluster check --apply, for better user experience.

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
• No code

Code changes

• Has exported function/method change
• Has exported variable/fields change
• Has interface methods change
• Has persistent data change

Side effects

• Possible performance regression
• Increased code complexity
• Breaking backward compatibility

Related changes

• Need to cherry-pick to the release branch
• Need to update the documentation

[codecov-commenter]

Codecov Report

Merging #550 into master will increase coverage by 11.08%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           master     #550       +/-   ##
===========================================
+ Coverage   39.84%   50.92%   +11.08%     
===========================================
  Files         201      220       +19     
  Lines       14847    15875     +1028     
===========================================
+ Hits         5916     8085     +2169     
+ Misses       8072     6688     -1384     
- Partials      859     1102      +243     

Flag	Coverage Δ
#coverage	50.92% <ø> (+11.08%)	⬆️

Impacted Files	Coverage Δ
...o/src/github.com/pingcap/tiup/server/store/sync.go	28.57% <0.00%> (-28.58%)	⬇️
....com/pingcap/tiup/pkg/cluster/clusterutil/retry.go	50.00% <0.00%> (-17.50%)	⬇️
go/src/github.com/pingcap/tiup/cmd/mirror.go	39.95% <0.00%> (-6.89%)	⬇️
...ingcap/tiup/components/playground/instance/tidb.go	85.36% <0.00%> (-5.34%)	⬇️
...ingcap/tiup/components/playground/instance/tikv.go	75.00% <0.00%> (-4.32%)	⬇️
...cap/tiup/components/playground/instance/tiflash.go	59.67% <0.00%> (-4.11%)	⬇️
go/src/github.com/pingcap/tiup/cmd/root.go	60.43% <0.00%> (-3.85%)	⬇️
...om/pingcap/tiup/components/cluster/command/root.go	43.03% <0.00%> (-2.24%)	⬇️
...m/pingcap/tiup/components/playground/playground.go	26.70% <0.00%> (-1.91%)	⬇️
.../pingcap/tiup/components/playground/instance/pd.go	75.43% <0.00%> (-1.23%)	⬇️
... and 113 more

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[ti-srebot]

/run-all-tests

[ti-srebot]

@hhkbp2 merge failed.