Show a more informative error when policy caption/description input isn't valid XML.
Show a more informative error when policy caption/description input isn't valid XML.

BUG=grit-i18n:1
TEST=build target policy_templates from chromium having a policy whose description contains "&". The error message should point to it.

Review URL: http://codereview.chromium.org/9113058/

lhopps/grit-i18n,sevansahumIlovemuhammad/grit-i18n,korusdipl/grit-i18n,lhopps/grit-i18n,korusdipl/grit-i18n,willbittner/grit-i18n,willbittner/grit-i18n,CrankWheel/grit-i18n,qjw/grit-i18n,sevansahumIlovemuhammad/grit-i18n,skidzen/grit-i18n,joisig/grit-i18n,megamanfx/grit-i18n,qjw/grit-i18n,joisig/grit-i18n,metasmile/grit-i18n,invalidstatement/grit-i18n,skidzen/grit-i18n,invalidstatement/grit-i18n,metasmile/grit-i18n,megamanfx/grit-i18n