http://userguide.icu-project.org/strings
Regarding the following statement in the release notes the U_STRING_NOT_TERMINATED_WARNING error need to be handle the same way as U_BUFFER_OVERFLOW_ERROR.
https://github.com/brianmario/charlock_holmes/blob/master/ext/charlock_holmes/converter.c#L29

"Some functions used to set an overflow error code even when only the terminating NUL did not fit into the output buffer. These functions now set UErrorCode to U_STRING_NOT_TERMINATED_WARNING rather than to U_BUFFER_OVERFLOW_ERROR."

Or is there any reason why the U_STRING_NOT_TERMINATED_WARNING should raise an error?