fix panic when cant openDB on tiup-bench

[nexustar]

What problem does this PR solve?

when cannot connect to DB, tiup-bench panic . fix #1548

What is changed and how it works?

instead of panic, just exit(1) and warning

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

Release notes:

NONE

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• AstroProfundis

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[codecov-commenter]

Codecov Report

Merging #1557 (5662d19) into master (2e089a2) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1557   +/-   ##
=======================================
  Coverage   26.24%   26.24%           
=======================================
  Files         266      266           
  Lines       24001    24001           
=======================================
  Hits         6298     6298           
  Misses      16883    16883           
  Partials      820      820           

Flag	Coverage Δ
integrate	15.81% <ø> (ø)
tiup	15.81% <ø> (ø)
unittest	22.78% <ø> (ø)

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[AstroProfundis]

/merge

[ti-chi-bot]

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

Commit hash: 5662d19