Improve WaitForPos errors, don't include Result struct in message

[rafer]

Description

Add more specific errors when WaitForPos receives an unexpected number of rows or columns. No longer prints the raw Result object (which isn't a particularly descriptive error for end users).

Immediate motivation is to make tests less brittle when Result object is modified (in downstream forks).

Related Issue(s)

N/A

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 68.61%. Comparing base (7997d1e) to head (f46bec4).
Report is 12 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #15962      +/-   ##
==========================================
+ Coverage   68.47%   68.61%   +0.13%     
==========================================
  Files        1562     1562              
  Lines      197062   198642    +1580     
==========================================
+ Hits       134933   136289    +1356     
- Misses      62129    62353     +224     

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[dbussink]

@rafer I think we should actually fix the error message generated here. I don't think it's very clear what this is in the first place. The error message does not seem very user friendly if it contains &{[] 0 0 [[]] 0 }. I don't think that's meaningful in any way for a Vitess user?

Something along these lines maybe, so instead of:

		case len(qr.Rows) > 1 || len(qr.Rows[0]) != 3:
			return fmt.Errorf("unexpected result: %v", qr)

We do something like:

		case len(qr.Rows) > 1:
			return fmt.Errorf("more rows received than expected, got %v instead of 1", len(qr.Rows))
		case len(qr.Rows[0]) != 3:
			return fmt.Errorf("more columns received than expected, got %v instead of 3", len(qr.Rows[0]))

[dbussink]

Alternatively, just printing qr.Rows instead of the whole qr already makes it look a lot better as well, since rows have a proper .String() implementation for debugging.

[rafer]

Update 👍

[mattlord]

Thanks, @rafer ! ❤️

[mattlord]

Super nitty, but we could use %d instead of %v here.