externalstore: Fix insert*() return docs and remove redundant checks

externalstore: Fix insert*() return docs and remove redundant checks

ExternalStorage::insertWithFallback is reported to return false on
failure, but it doesn't. It has a single exit point, and return value is
checked with strlen(), so actually it can only return the URL or throw.
Thus, update any related doc and remove a redundant check from code
calling insertToDefault.

Change-Id: Ic95c3aed19118b987aef105f8077d55558f39127