Improve error messages

[deepthi]

Feature Description

Right now we produce errors like "no valid tablet", which are accurate but not very helpful. Sometimes the error messages also contain references to internal functions like execInsertUnsharded.

• [x] Remove vitess-internal texts from error messages (e.g. function names)
• [x] Every error must have some error code, and should closely match the type of error defined by MySQL. If possible also return root cause.
• [x] Document all error messages and root causes https://github.com/vitessio/website/pull/750

Use Case(s)

VTAdmin Usability

[derekperkins]

This would be very useful to support on the Go driver side. We currently have a ton of text parsing to recreate the original proto error struct.

[GuptaManan100]

Is there something pending on this issue? We are now generating documentation for the website automatically.

[harshit-gangal]

The scoped work is completed.