Fix error return values in WikiPage::getSection and setText

[Xymph]

Commit aeaa486 introduced checks in getSection() for undefined sections that return false if not found. This is inconsistent with other methods (e.g. getText(), Wikimate::download() and many Wikifile methods) which return null to indicate error (and otherwise a string or array). Only boolean methods should return false upon error, I think.

Also, recent commit 0a00740 returns a null upon error in boolean method setText(), which should be false.

The getSection change is somewhat API breaking -- the difference is only noticeable when testing with '===' and '!==' but I did that in some of my client scripts -- so it should go in just before we bump to v1.0.0.