cluster: update error message of tiflash not ready

[AstroProfundis]

What problem does this PR solve?

The status TiFlash reports could be Ready before it becomes Running, so the current error message could be confusing.

What is changed and how it works?

Adjust the error message.

Check List

Tests

• Manual test (add detailed scripts or steps below)

Release notes:

NONE

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• nexustar

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[codecov-commenter]

Codecov Report

Merging #1618 (c9168c1) into master (01843be) will increase coverage by 35.11%.
The diff coverage is 0.00%.

@@             Coverage Diff             @@
##           master    #1618       +/-   ##
===========================================
+ Coverage   15.63%   50.75%   +35.11%     
===========================================
  Files         150      291      +141     
  Lines       15169    26559    +11390     
===========================================
+ Hits         2372    13480    +11108     
+ Misses      12280    11051     -1229     
- Partials      517     2028     +1511     

Flag	Coverage Δ
cluster	43.89% <0.00%> (?)
dm	26.24% <0.00%> (?)
integrate	50.75% <0.00%> (+35.11%)	⬆️
playground	13.41% <0.00%> (?)
tiup	15.63% <0.00%> (ø)
unittest	∅ <ø> (∅)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
pkg/cluster/spec/tiflash.go	51.50% <0.00%> (+51.50%)	⬆️
components/dm/command/upgrade.go	81.81% <0.00%> (ø)
components/cluster/command/scale_in.go	91.66% <0.00%> (ø)
components/playground/instance/pump.go	0.00% <0.00%> (ø)
pkg/cluster/ansible/inventory.go	0.00% <0.00%> (ø)
components/dm/spec/bindversion.go	100.00% <0.00%> (ø)
components/playground/main.go	44.96% <0.00%> (ø)
components/playground/monitor.go	85.18% <0.00%> (ø)
components/playground/instance/instance.go	51.72% <0.00%> (ø)
pkg/cluster/task/update_topology.go	70.58% <0.00%> (ø)
... and 235 more

---

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[AstroProfundis]

/merge

[ti-chi-bot]

This pull request has been accepted and is ready to merge.

Commit hash: c9168c1