Improve errors and diagnostics

[martinbonnin]

I had a leftover (wrong) @JsonAdapter annotation:

@JsonClass(generateAdapter = true, generator = "sealed:kind")
data class MyClass(val prop1: String, val prop2: Int)

The error in this case is:

> Task kspKotlin FAILED
e: [ksp] java.lang.IllegalStateException: Must be a sealed class!

It would be nice if the error included the source location where it is.

[ZacSweers]

Hmm, it should be reporting that. I'll double check