Fix printing of SwMatchError after introduction of logical channels

the interpret_sw() method was moved from RuntimeState to RuntimeLchan
in Change-Id I7aa994b625467d4e46a2edd8123240b930305360 - but the code
in pySim/exceptions.py was not adjusted accordingly.

Change-Id: I0614436c99c6a6ebc22c4dc14fb361c5f5f16686

pySim/exceptions.py

@@ -53,8 +53,8 @@ class SwMatchError(Exception):
         self.rs = rs
 
     def __str__(self):
-        if self.rs:
-            r = self.rs.interpret_sw(self.sw_actual)
+        if self.rs and self.rs.lchan[0]:
+            r = self.rs.lchan[0].interpret_sw(self.sw_actual)
             if r:
                 return "SW match failed! Expected %s and got %s: %s - %s" % (self.sw_expected, self.sw_actual, r[0], r[1])
         return "SW match failed! Expected %s and got %s." % (self.sw_expected, self.sw_actual)