This option effects
pse.plugins.request.form object. pse.plugins.request.form is a
dictionary-like object that contains the GET and POST arguments for
the currently requested page.
Setting FormKeyError to Yes will cause
pse.plugin.request.form to behave like a normal dictionary
(raising a KeyError) if you try to access a non-existent
key.
Setting FormKeyError to No will cause
pse.plugin.request.form to return an empty string ('')
if an invalid key is used. has_key(), keys(), etc. will
still behave normally. By default, it is set to No.
See section 5.2.3 for more information.