Emit hard error when differentiation visits unsupported statement

[efremale]

Currently, if we Visit unsupported statement (e.g. try-catch, range for loop), it is simply cloned without changes and a warning is emitted.

Should we instead emit a hard error and not produce the derivative?

This is somewhat harder to achieve now.
How should the error be promoted in the visitor?
If we return null StmtDiff whenever we meet an unsupported statement, we would have to add a lot of checks for null.
We cannot throw an exception sice clang disables them.
We can set some bool flag and keep visiting function and check for the error flag in the end, but it is inefficient and error-prone.