Unable to view extensions page after attempting to uninstall extension in use

Unable to access civicrm/admin/extensions?reset=1

Steps to reproduce on 4.1.3

1. Install payment processor extension
2. Configure payment processor
3. Disable extension (this works OK)
4. Uninstall extension

At this point you'll see a Fatal "extension in use" error, but the extension files are removed anyway.

From here it's impossible to view the extensions page, because CRM_Core_Extensions_Extension::_parseXML() can't access the info.xml file.

1. _parseXML() should cope with that better.
2. If we're not allowed to uninstall the extension, we should keep the extension files around