Update error meesage for TfsWorkItemTypeValidatorTool

[MrHinsh]

Hey @satano can we get an update to the message as its blocking folks. Its ok to just run and loose data for most folks...

Can we update to:

[06:34:12 ERR] [16.2.11-Preview.1] Some work item types or their fields are not present in the target system (see previous logs). Either add these fields into target work items, or map source fields to other target fields in options (FieldMappings).  You can turn off this val
[06:34:12 INF] [16.2.11-Preview.1] If you have some field mappings defined for validation, do not forget also to configure proper field mapping in FieldMappingTool so data will preserved during migration. 

• Update error message to explain how to turn this off
• Update /docs/reference/tools/work-item-type-validator-tool/ with a clear explanation of the intent and outcomes.
• Add sample configuration to appsettings.json (it probably should be a separate file but its all mushed together for now)

Having the instructions in the error message to tell people how to go turn it off would be awesome.

[satano]

Sure, we can do this to make it more understandable for people. Just my notes.

1. Is it really the case, that most people want "just run and loose data"? Maybe yes. But from my own case the problem was, that I even did not know that I lost some data until some later time.
2. I moved validation of reflected work item ID into this validation. Before, this was hard validation, meaning if this field did not exist, the migration immediately failed. Now if somebody will turn off this validator, reflected work item ID will not be validated either, which may be a problem.
3. I like this validatoin, because it really helped me in next project I am going to validate. Bus I still do not consider it finished. I would like to put FieldMappingTool into the process, so the validation will look into its configuration and if there are relevant mappings, use them. And so do not trigger ewarnings about missing fields and field values, if they are mapped. Even the log messages can be more helpful, because we can log out proper configuration for field mapping tool which user will just copy and paste. Of course it will not be possible to handle all cases and mapping types, byt still, it can be helpful.

I can implement points 2 and 3, but not now, but two weeks later. My holiday just started.

[satano]

Based on the tag in Git, all this is still in preview. I do not know when you want to release stable version, but could you possibly wait until the things I mentioned are implemented? Because that may still change the configuration and so it can be annoying if it will change between stable versions. As it is preview, I consider configuration itself as a preview.

[MrHinsh]

I made a bunch of updates to the docs, and added a better wrapper message to the validator in the work item processor.