Refactor and correct divergence warning

Author: eigenfoo

pymc3/step_methods/hmc/base_hmc.py

@@ -171,18 +171,19 @@ def warnings(self):
         # Generate a global warning for divergences
         n_divs = self._num_divs_sample
         if n_divs and self._samples_after_tune == n_divs:
-            msg = ('The chain contains only diverging samples. The model is '
-                   'probably misspecified.')
-            warning = SamplerWarning(
-                WarningType.DIVERGENCES, msg, 'error', None, None, None)
-            warnings.append(warning)
+            message = ('The chain contains only diverging samples. The model '
+                       'is probably misspecified.')
+        elif n_divs == 1:
+            message = ('There was 1 divergence after tuning. Increase '
+                       '`target_accept` or reparameterize.')
         elif n_divs > 0:
             message = ('There were %s divergences after tuning. Increase '
                        '`target_accept` or reparameterize.'
                        % n_divs)
-            warning = SamplerWarning(
-                WarningType.DIVERGENCES, message, 'error', None, None, None)
-            warnings.append(warning)
 
+        warning = SamplerWarning(
+            WarningType.DIVERGENCES, message, 'error', None, None, None)
+        warnings.append(warning)
         warnings.extend(self.step_adapt.warnings())
+
         return warnings