dice: edge case cleanup

Python 3.5 changed `compile()` (which `eval_equation()` uses, far down
the call stack) to raise `ValueError` in the case that used to raise a
`TypeError`, so that block isn't necessary any more.

See https://docs.python.org/3.8/library/functions.html#compile and
#1386 (comment)

Input that doesn't contain any dice expressions could still be processed
and wind up at the wrong error handler; added an early-return for that.
(An expression without dice rolls should be evaluated using `.calc`. And
yes, I'm aware that this could be someone's spacebar heater. Tough!)

The tests also now specifically cover an expression with no number of
dice (to verify that the default of 1 die works), and cases where the
input contains one or more valid dice expression(s) but has invalid
syntax somewhere else in the string.

sopel/builtins/dice.py

@@ -241,9 +241,10 @@ def _roll_dice(dice_expression: str) -> DicePouch:
 @plugin.command('roll', 'dice', 'd')
 @plugin.priority("medium")
 @plugin.example(".roll", "No dice to roll.")
-@plugin.example(".roll 65(2)",
+@plugin.example(".roll 2d6+4^2&",
                 "I don't know how to process that. "
                 "Are the dice as well as the algorithms correct?")
+@plugin.example(".roll 65(2)", "I couldn't find any valid dice expressions.")
 @plugin.example(".roll 1d0", "I don't have any dice with 0 sides.")
 @plugin.example(".roll -1d6", "I can't roll -1 dice.")
 @plugin.example(".roll 3d6v-1", "I can't drop the lowest -1 of 3 dice.")
@@ -262,7 +263,7 @@ def _roll_dice(dice_expression: str) -> DicePouch:
     ".roll {}d1".format(MAX_DICE + 1), 'I only have {}/{} dice.'.format(MAX_DICE, MAX_DICE + 1))
 @plugin.example(".roll 1d1 + 1d1", '1d1 + 1d1: (1) + (1) = 2')
 @plugin.example(".roll 1d1+1d1", '1d1+1d1: (1)+(1) = 2')
-@plugin.example(".roll 1d6 # initiative", r'1d6: \(\d\) = \d', re=True)
+@plugin.example(".roll d6 # initiative", r'd6: \(\d\) = \d', re=True)
 @plugin.example(".roll 2d20v1+2 # roll with advantage", user_help=True)
 @plugin.example(".roll 2d10+3", user_help=True)
 @plugin.example(".roll 1d6", user_help=True)
@@ -292,6 +293,10 @@ def roll(bot: SopelWrapper, trigger: Trigger):
     arg_str = arg_str_raw.replace("%", "%%")
     arg_str = re.sub(dice_regexp, "%s", arg_str)
 
+    if not dice_expressions:
+        bot.reply("I couldn't find any valid dice expressions.")
+        return
+
     try:
         dice = [_roll_dice(dice_expr) for dice_expr in dice_expressions]
     except DiceError as err:
@@ -315,12 +320,6 @@ def _get_pretty_str(dice: DicePouch) -> str:
 
     try:
         result = eval_equation(eval_str)
-    except TypeError:
-        bot.reply(
-            "The type of this equation is, apparently, not a string. "
-            "How did you do that, anyway?"
-        )
-        return
     except ValueError:
         # As it seems that ValueError is raised if the resulting equation would
         # be too big, give a semi-serious answer to reflect on this.