throw real exceptions instead of die() when it makes sense

@@ -1687,10 +1687,10 @@ Casper.prototype.wait = function wait(timeout, then) {
     this.checkStarted();
     timeout = ~~timeout;
     if (timeout < 1) {
-        this.die("wait() only accepts a positive integer > 0 as a timeout value");
+        throw new CasperError("wait() only accepts a positive integer > 0 as a timeout value");
     }
     if (then && !utils.isFunction(then)) {
-        this.die("wait() a step definition must be a function");
+        throw new CasperError("wait() a step definition must be a function");
     }
     return this.then(function _step() {
         this.waitStart();
@@ -1730,10 +1730,10 @@ Casper.prototype.waitFor = function waitFor(testFx, then, onTimeout, timeout) {
     this.checkStarted();
     timeout = timeout ? timeout : this.options.waitTimeout;
     if (!utils.isFunction(testFx)) {
-        this.die("waitFor() needs a test function");
+        throw new CasperError("waitFor() needs a test function");
     }
     if (then && !utils.isFunction(then)) {
-        this.die("waitFor() next step definition must be a function");
+        throw new CasperError("waitFor() next step definition must be a function");
     }
     return this.then(function _step() {
         this.waitStart();
@@ -1742,7 +1742,8 @@ Casper.prototype.waitFor = function waitFor(testFx, then, onTimeout, timeout) {
         var interval = setInterval(function _check(self, testFx, timeout, onTimeout) {
             if ((new Date().getTime() - start < timeout) && !condition) {
                 condition = testFx.call(self, self);
-            } else {
+                return;
+            }
             self.waitDone();
             if (!condition) {
                 self.log("Casper.waitFor() timeout", "warning");
@@ -1759,7 +1760,6 @@ Casper.prototype.waitFor = function waitFor(testFx, then, onTimeout, timeout) {
                 }
             }
             clearInterval(interval);
-            }
         }, 100, this, testFx, timeout, onTimeout);
     });
 };