Add hash check on document read/write (#890)

* Add sha256sum function
* Add hash check to document class
* Make sha256sum test more thorough
* Handle exceptions in the sha256sum function directly
* Update test coverage
* Clarify title on dialog box
* Don't write blank lines in makeAlert

novelwriter/guimain.py

@@ -1121,6 +1121,7 @@ class GuiMain(QMainWindow):
         can be either a string or an array of strings.
         """
         if isinstance(theMessage, list):
+            theMessage = list(filter(None, theMessage))
             popMsg = "<br>".join(theMessage)
             logMsg = theMessage
         else: