[calc] Add guards to pow and mul functions to avoid hangs.

- Fixes sopel-irc#538.

- Removed the drivel from calc when exceptions are raised. Printing the actual
  error might not be pretty, but at least it's not completely useless.

calc.py

@@ -23,7 +23,7 @@
 @commands('c', 'calc')
 @example('.c 5 + 3', '8')
 def c(bot, trigger):
-    """Google calculator."""
+    """Evaluate some calculation."""
     if not trigger.group(2):
         return bot.reply("Nothing to calculate.")
     # Account for the silly non-Anglophones and their silly radix point.
@@ -32,10 +32,9 @@ def c(bot, trigger):
         result = str(eval_equation(eqn))
     except ZeroDivisionError:
         result = "Division by zero is not supported in this universe."
-    except Exception:
-        result = ("Sorry, I can't calculate that with this command. "
-                  "I might have another one that can. "
-                  "Use .commands for a list.")
+    except Exception as e:
+        result = "{error}: {msg}".format(
+                error=type(e), msg=e)
     bot.reply(result)