Fail VTBackup early when we cannot check replication status or when MySQL has failed

[frouioui]

Description

Related Issue(s)

Checklist

• "Backport to:" labels have been added if this change should be back-ported to release branches
• If this change is to be back-ported to previous releases, a justification is included in the PR description
• Tests were added or are not required
• Did the new or modified tests pass consistently locally and on CI?
• Documentation was added or is not required

Deployment Notes

[vitess-bot]

Review Checklist

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[codecov]

Codecov Report

Attention: Patch coverage is 0% with 25 lines in your changes missing coverage. Please review.

Project coverage is 67.48%. Comparing base (63dfb9e) to head (ee310c8).

Files with missing lines	Patch %	Lines
go/cmd/vtbackup/cli/vtbackup.go	0.00%	17 Missing ⚠️
go/vt/mysqlctl/mysqld.go	0.00%	8 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #17356      +/-   ##
==========================================
- Coverage   67.51%   67.48%   -0.03%     
==========================================
  Files        1580     1580              
  Lines      253805   253822      +17     
==========================================
- Hits       171358   171303      -55     
- Misses      82447    82519      +72     

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