Clarify the situation when generated extension property causes ClassCast or NPE exceptions

[koperagen]

Now if compiler plugin or jupyter integration has a bug and generated property returns type that mismatches runtime, we get vague ClassCast or NPE. This PR tries to give a little bit more info about it and propose a workaround

[Jolanrensen]

If I'm correct, you fixed it just for convert.to, right?
However the ClassCastException can occur any time a value from a column is accessed, like:

val ColumnsContainer<Marker>.a get() = this["a"] as DataColumn<String>
val DataRow<Marker>.a get() = this["a"] as String

df.filter { a != "something" } // throws ClassCastException if `a` does not contain Strings
val values = df.a.values()
values[0] // throws ClassCastException if `a` does not contain Strings (not sure if this can be avoided)

[koperagen]

For convert.with
Yes, there are other cases, but for example this one is a bit better. Because this["a"] as String is checked cast, so it will fail right there and with sensible stack trace.

val DataRow<Marker>.a get() = this["a"] as String
df.filter { a != "something" } // throws ClassCastException if `a` does not contain Strings

When DataColumn accessors are called, it's unchecked cast and value can travel quite a bit before library "extract" value from the column. So stack trace and exception message are less helpful - i think convert with is popular example of such operation

[Jolanrensen]

@koperagen I see :) Well, then convert.with throwing the new exception is a good start :) We'll likely need to add it in other places as well, but maybe that can be done later after we discover where else it pops up.