[REQ] [KOTLIN] Don't use TODO() function provided by Kotlin

[toXel]

The use of the Kotlin function TODO() can IMO be dangerous sometimes. For example here:

openapi-generator/modules/openapi-generator/src/main/resources/kotlin-client/infrastructure/ApiClient.kt.mustache

Lines 60 to 63 in 4d28593

	return when(mediaType) {
	JsonMediaType -> Serializer.moshi.adapter(T::class.java).fromJson(bodyContent)
	else -> TODO("responseBody currently only supports JSON body.")
	}

When mediaType is not application/json the app or program, in which the client is used, just crashes. AFAIK there is no way to catch these exceptions without modifying the ApiClient class.

Describe the solution you'd like

Replace TODO() calls with catchable exceptions or return null and add an // TODO comment. I've tested both attempts and they worked without crashing the app.

[jimschubert]

👍 I think the custom exception option would be the better approach than an implementer comment and returning null.

[jimschubert]

I opened #3611 to change TODO() to throw UnsupportedOperationException. What are your thoughts on this exception type rather than some custom exception?

[toXel]

I think that's a great solution. Thank you for the PR! I hope it'll be merged soon.

[jimschubert]

I've merged the PR. Please keep an eye out for the change in the latest snapshot.

Feel free to reopen the issue if the fix needs any adjustments.