maintenance: Allow having a nicer error message if an extension isn't enabled

maintenance: Allow having a nicer error message if an extension isn't enabled

Maintenance scripts can be invoked regardless of whether an extension is
enabled on a wiki or not. On wiki farms where some wikis may have an
extension or not, this can potentially be rather confusing. Especially
when the script bails out with a fatal class missing error or a database
table missing.

This allows maintenance scripts to specify that they require an
extension by calling:
 $this->requireExtension( 'ExtensionName' );
in the script's constructor.

Bug: T141531
Change-Id: Icfbf063bb9c9ac9e55c3a5a8ed815528a2c1ce1e