The change from using strings to objects for passing errors across the language barrier broke the custom java package mode of gombile bind. Fix it and add a runtime test to make sure it won't happen again. Fixes golang/go#16262 Change-Id: Ia7f8afb79556798056f0755758052190081a2dbb Reviewed-on: https://go-review.googlesource.com/24800 Reviewed-by: Hyang-Ah Hana Kim <hyangah@gmail.com>