error handling "feature/content filter error architecture"

[danielweck]

Origin:
readium/readium-sdk#239

NYPL-Simplified URMS example usage:
NYPL-Simplified/readium-sdk@develop_v26...d-i-t-a:develop_v26_urms#diff-3eaa05572cddd39a2fb0ee1f452b735f

Cherry-pick from the feature branch:
https://github.com/d-i-t-a/readium-sdk/commit/5c3bc538eb2756c236a582b030053fbb7450361e

URMS usage (search for HandleContentFilterError):
https://github.com/d-i-t-a/readium-sdk/commit/83850410fe5a39c5c6f876dbef2b774cb0b55049

[danielweck]

Also see:
NYPL-Simplified/readium-sdk#2