Fixes a crash that could occur cleaning up videopress markup if an expected match was not found.

Clarifies variable names and adds soem comments.
In a case where a match is not found the VideoPress markup is replaced with an empty string.
Log strings that fail to have a match.
Merge pull request #984 from wordpress-mobile/issues/963-videopress-crash

Fixes a crash that could occur cleaning up videopress markup if an expec...