Wrap .sheet.cssRules access in try...catch.

[PaulJThompson]

Summary

Wrap accesses to .sheet.cssRules in a try...catch block. On Firefox, this access can cause a DOMException to be thrown ('SecurityError') which interrupts the entire process. Log-and-resume to avoid it.

This PR fixes/implements the following bugs/features

• node.sheet.cssRules accessed outside of try-catch, so the thrown DOMException is uncaught #1673 Bug 1

Explain the motivation for making this change. What existing problem does the pull request solve?

The uncaught exception means that the entire html2canvas operation is interrupted and no image is produced. At least for our application, the cssRule can be safely ignored.

Test plan (required)

I'm still working on this. Adding a file into tests/reftests isn't working because the documents don't get cloned first before copying them.

Code formatting

Please make sure that code adheres to the project code formatting. Running npm run format will automatically format your code correctly.

Closing issues
closes #1673

[PaulJThompson]

@cschramm thanks for approving my PR. However, I don't have write access to this repos. Do you?

[cschramm]

No, I just wanted to express that I run into the same issue and that the solution seems fine to me.