Define some standard error types for use by lang.* modules

[jclark]

At the moment, lang.* modules than return errors just return error, but they should return a specific type of error with specific details.

This could be pushed to later

[jclark]

This depends on #227.

[jclark]

With distinct error types from #413, we are now ready to do this.

[jclark]

We should also define types for errors generated by language panics.

[jclark]

We can categorize the langlib functions that return errors as follows:

• Functions like int:fromString. These start with from and each function creates a specific type of value from a specific type of input. This covers most cases.
• Functions that construct a value belonging to a type specified by a typedesc argument. These are all in lang.value: cloneWithType, fromJsonWithType, fromJsonStringWithType
• value:mergeJson
• value:ensureType (this is also generated by check expressions in some cases and so relates to Need distinct error type for language errors from json and lax typing #806)

[jclark]

We are handling panics in #807.