LPS-43148 Move frequently used import "FileExtensionException" to common/init - no logic change
LPS-43148 Update exception-handling in UploadImageAction

Similar pattern can be found in the EditFileEntryAction#handleUploadException, however, we should not add any error messages to SessionErrors when the action is "ADD_TEMP", as it is generated by a file-upload, not a full form submit. If we added the exceptions to both JSON and the SessionErrors, the user could get unexpected UI errors - coming from a previous action -, upon closing (hiding) and reopening the Logo Edito pop-up.
LPS-43148 Update logo_editor.js #1 - exception-handling
LPS-43148 Fix possible NPE in UploadImageAction

Steps to reproduce:
1- Open Logo Editor
2- Click on 'Save'
--> NPE occurs in the log, 'Portlet temporary unavailable' message appears on the UI
LPS-43148 Add form-validator to image_uploader view
LPS-43148 Update logo_editor.js #2 - validation
LPS-43148 Logo Editor still generates FileExtensionException, even if the user uploads a file with an extension allowed
LPS-43148 Update logo_editor.js #3 - image preview
LPS-43148 Always clear the current render parameter pool for the image_uploader (195) portlet

Without this, there would be duplicated parameters ("cmd", "struts_action", "tempImageFileName" etc.) in the resourceURL (= "previewURL") upon closing (hiding) & reopening the Logo Editor view after a successfull tempImage upload
LPS-43148 Do not cache Logo Editor window
LPS-44148 Autofocus on the file input field in POP-UP window state as well
LPS-43148 Fix misspell in lang message