Main: Tweak error messages
Main: Tweak error messages

When a platform could not be loaded, the error message referred to
the Android SDK, but nowadays with the loadable platforms this is
not correct any more, and we need to use a more general error
message.

pozdnyakov/crosswalk-app-tools,crosswalk-project/crosswalk-app-tools,baleboy/crosswalk-app-tools,shawngao5/crosswalk-app-tools,shawngao5/crosswalk-app-tools,davidmaxwaterman/crosswalk-app-tools,pozdnyakov/crosswalk-app-tools,hgl888/crosswalk-app-tools,davidmaxwaterman/crosswalk-app-tools,crosswalk-project/crosswalk-app-tools,hgl888/crosswalk-app-tools,baleboy/crosswalk-app-tools,baleboy/crosswalk-app-tools,crosswalk-project/crosswalk-app-tools