Updated some typos

Fixed some incorrect variable references.
Makes the error names consistent
Don't refer to flaskext in docs
Merge pull request #886 from talam/patch-1

Updated some typos
Merge pull request #890 from seancron/documentation-fix

Documentation fix: Makes the error names consistent
Merge pull request #891 from ThiefMaster/docs-flaskext

Don't refer to flaskext in app.extensions docs
Clear exceptions when pushing a context

Fixes #882
Merge branch '0.10-maintenance'
Merge branch 'master', remote-tracking branch 'upstream/master'