Better error handling

[thomthom]

We should present an error message when importing junk files such as this one: https://dl.dropboxusercontent.com/u/4791584/SketchUp/STL/luer.stl

Reference: http://forums.sketchup.com/t/stl-plugin-hung-up-at-importing-geometry/1793/6

[jimfoltz]

Also, if the stl file is less than 80 bytes, we get an Exception because file.read(4) returns nil, and then we try to call unpack on nil

https://github.com/SketchUp/sketchup-stl/blob/master/src/sketchup-stl/importer.rb#L161