Each error should be its own subtype

[edgarsi]

It is reasonable to rewrite error messages to whatever users find more understandable (e.g. less technical jargon, add use-case recommendations, translate to a different language). Catching DecodingError with a string description is not friendly to that, because descriptions can be revised in future changes. Imo the cleanest solution is to subclass. Maybe something like this:

sealed class DecodingError(params: Params): TomlException() {
    internal data class Params (
        val reason: String,
        val sourceValue: TomlValue,
        val targetType: KType,
        val cause: Throwable? = null
    )
    val reason = params.reason
    val sourceValue = params.sourceValue
    val targetType = params.targetType
    override val cause: Throwable? = params.cause
}

class NoValueForNonNullableParameter internal constructor(params: Params): DecodingError(params)

....
    throw NoValueForNonNullableParameter(DecodingError.Params(
        "no value found for non-nullable parameter '${constructorParameter.name}'",
        tomlMap,
        kType
        ))