Storage: Fix handling of string facts in update_fact()

This method was written to handle either a Fact object or a string
description of a fact. However, the string-to-Fact conversion happened
too late in the method, after `check_fact()` was called, which requires
a Fact object, leading to an error like:

    AttributeError: 'dbus.String' object has no attribute 'start_time'

This happened in particular when using the DBus UpdateFact method, which
always passes a string to `update_fact()`.

This commit ensures that the string-to-Fact conversion is done
immediately, so the rest of the method can just assume there is a Fact
method.

This fixes #671.

src/hamster/storage/storage.py

@@ -122,14 +122,14 @@ def get_fact(self, fact_id):
         return self.__get_fact(fact_id)
 
     def update_fact(self, fact_id, fact, start_time=None, end_time=None, temporary=False):
-        # better fail before opening the transaction
-        self.check_fact(fact)
-        self.start_transaction()
-        self.__remove_fact(fact_id)
         # to be removed once update facts use Fact directly.
         if isinstance(fact, str):
             fact = Fact.parse(fact)
             fact = fact.copy(start_time=start_time, end_time=end_time)
+        # better fail before opening the transaction
+        self.check_fact(fact)
+        self.start_transaction()
+        self.__remove_fact(fact_id)
         result = self.__add_fact(fact, temporary)
         if not result:
             logger.warning("failed to update fact {} ({})".format(fact_id, fact))