Fix logs when Mimic is not found

If Mimic was not found at all the LOG message would raise an exception,
this fixes that issue and cleans up Exception chain

mycroft/tts/mimic_tts.py

@@ -185,10 +185,14 @@ class MimicValidator(TTSValidator):
     def validate_connection(self):
         try:
             subprocess.call([BIN, '--version'])
-        except Exception:
-            LOG.info("Failed to find mimic at: " + BIN)
+        except Exception as err:
+            if BIN:
+                LOG.error('Failed to find mimic at: {}'.format(BIN))
+            else:
+                LOG.error('Mimic executable not found')
             raise Exception(
-                'Mimic was not found. Run install-mimic.sh to install it.')
+                'Mimic was not found. Run install-mimic.sh to install it.') \
+                from err
 
     def get_tts_class(self):
         return Mimic