Customizable in the preferences
Also improves handling of Zotero startup errors, adding the ability to customize the tooltiptext of the status bar error icon and to have ZoteroPane.toggleDisplay() run a customizable error function (it no longer opens an empty and broken Zotero pane). For example, a missing Z directory now prompts the user to locate the directory.
Various code parts now check for the Zotero object and Zotero.initialized before trying to do stuff, which should cut down on redundant error lines in the console and generally make things cleaner.
Refs #542, Better icons for preferences window -- need icon for Advanced prefpane