Add limited set of sanity checks for usage of appcache in webapps.

I have the feeling that it would be better to improve installation and runtime code to expose error messages instead of doing nothing -or- dumping almost meaningless error code in b2g stdout.
Another middle step could be to start sharing sanity check methods between apps platform code and the simulator.
If we continue on that road one another farer step could be to start implementing checks in simulator in order to eventually upstream them in this apps platform code, that's what I'm trying to do in this pull request with a first module that focuses on checking appcache related failures.

If you like this approach, I'd like to continue and factor out of simulator.js all validation code, especially `_validateNameIcons`, `_validateWebAPIs`. And also try to move prosthesis validation code hosted in simulator actor (`onValidateManifest` and `_validateManifestPermissions`) to the firefox addon [if that's reasonnably possible].