present: return an error for a bad link instead of panic
present: return an error for a bad link instead of panic

Change-Id: Ic41b2794268b6399bda01febafb85852f6634198
Reviewed-on: https://go-review.googlesource.com/44690
Reviewed-by: Andrew Gerrand <395a7d33bec8475c9b83b7d440f141bcbd994aa5@golang.org>

syreclabs/go-tools,golang/tools,syreclabs/go-tools,syreclabs/go-tools,syreclabs/go-tools,golang/tools,golang/tools,syreclabs/go-tools,golang/tools,syreclabs/go-tools,golang/tools