Improve assert-throws? macro

Since `assert-throws?` throws an exception inside the try regardless of
if the body throws, if the body *doesn't* throw, you get a sort of
strange error message:

```
RuntimeException: Assert failed Expected message: exception! but got
Assert failed Expected a <type pixie.stdlib.RuntimeException>
exception: exception!
```

I fixed this, improved the messages a bit, and also added some extra
arities for just checking that an expression throws, and also just
checking the class of the exception, instead of always the class and the
message.

Author: justinj

pixie/test.pxi

@@ -61,15 +61,25 @@
           yr# ~y]
      (assert (= xr# yr#) (str (show '~x xr#) " != " (show '~y yr#)))))
 
-(defmacro assert-throws? [klass msg body]
-     `(try 
-       ~body 
-       (assert false (str "Expected a " ~klass " exception: " ~msg))
-       (catch e#
-         (assert (= (type e#) ~klass) 
-                 (str "Expected exception of class " ~klass " but got " (type e#)))
-         (assert (= (ex-msg e#) ~msg) 
-                 (str "Expected message: " ~msg " but got " (ex-msg e#))))))
+(defmacro assert-throws?
+  ([body]
+   `(let [exn# (try (do ~body nil) (catch e# e#))]
+      (assert (not (nil? exn#))
+              (str "Expected " (pr-str (quote ~body)) " to throw an exception"))
+      exn#))
+  ([klass body]
+   `(let [exn# (assert-throws? ~body)]
+     (assert (= (type exn#) ~klass)
+             (str "Expected " (pr-str (quote ~body))
+                  " to throw exception of class " (pr-str ~klass)
+                  " but got " (pr-str (type exn#))))
+     exn#))
+  ([klass msg body]
+   `(let [exn# (assert-throws? ~klass ~body)]
+     (assert (= (ex-msg exn#) ~msg)
+             (str "Expected " (pr-str (quote ~body))
+                  " to throw exception with message " (pr-str ~msg)
+                  " but got " (pr-str (ex-msg exn#)))))))
 
 (defmacro assert [x]
   `(let [x# ~x]