Silently failing when package files are inaccessible

[pmcharrison]

It is possible for a client computer to successfully install psychTestR, but not have sufficient privileges to access package files using system.file(). Currently psychTestR silently fails during such cases - in particular, the Javascript helper scripts fail to load, so the user cannot progress to successive pages in the test. We should do two things:

• Add mustWork = TRUE to calls to system.file();
• Add an automatic permissions check to ensure that the required package files are readable.