Improve error understandability when exception thrown during schema generation

[Chuckame]

Is your feature request related to a problem? Please describe.
As seen in #264, the error weren't explicit at all, especially when you don't know how works the kotlinx serialization library.

Here an example of error:

com.github.avrokotlin.avro4k.internal.AvroSchemaGenerationException: Polymorphic descriptor 'ContextDescriptor(kClass: class kotlinx.serialization.KSerializer, original: kotlinx.serialization.Polymorphic(type: kotlin.String, value: kotlinx.serialization.Polymorphic<KSerializer>))' must have at least one possible schema

Describe the solution you'd like
We have to add the path of this error, to be able to locate well the error as it could be deep in the classes hierarchy.