Make sure syntax errors in regular expressions raise conforming exceptions

Just letting the error from new RegExp through creates an exception missing the
pos/loc/raisedAt properties.

Issue #82
Issue #81

acorn.js

@@ -767,7 +767,13 @@
     // here (don't ask).
     var mods = readWord1();
     if (mods && !/^[gmsiy]*$/.test(mods)) raise(start, "Invalid regexp flag");
-    return finishToken(_regexp, new RegExp(content, mods));
+    try {
+      var value = new RegExp(content, mods);
+    } catch (e) {
+      if (e instanceof SyntaxError) raise(start, e.message);
+      raise(e);
+    }
+    return finishToken(_regexp, value);
   }
 
   // Read an integer in the given radix. Return null if zero digits

index.html

@@ -532,7 +532,13 @@ since a '/' inside a '[]' set does not end the expression.</p>             </td>
     <span class="o">++</span><span class="nx">tokPos</span><span class="p">;</span></pre></div>             </td>           </tr>                               <tr id="section-70">             <td class="docs">               <div class="pilwrap">                 <a class="pilcrow" href="#section-70">&#182;</a>               </div>               <p>Need to use <code>readWord1</code> because '\uXXXX' sequences are allowed
 here (don't ask).</p>             </td>             <td class="code">               <div class="highlight"><pre>    <span class="kd">var</span> <span class="nx">mods</span> <span class="o">=</span> <span class="nx">readWord1</span><span class="p">();</span>
     <span class="k">if</span> <span class="p">(</span><span class="nx">mods</span> <span class="o">&amp;&amp;</span> <span class="o">!</span><span class="sr">/^[gmsiy]*$/</span><span class="p">.</span><span class="nx">test</span><span class="p">(</span><span class="nx">mods</span><span class="p">))</span> <span class="nx">raise</span><span class="p">(</span><span class="nx">start</span><span class="p">,</span> <span class="s2">&quot;Invalid regexp flag&quot;</span><span class="p">);</span>
-    <span class="k">return</span> <span class="nx">finishToken</span><span class="p">(</span><span class="nx">_regexp</span><span class="p">,</span> <span class="k">new</span> <span class="nb">RegExp</span><span class="p">(</span><span class="nx">content</span><span class="p">,</span> <span class="nx">mods</span><span class="p">));</span>
+    <span class="k">try</span> <span class="p">{</span>
+      <span class="kd">var</span> <span class="nx">value</span> <span class="o">=</span> <span class="k">new</span> <span class="nb">RegExp</span><span class="p">(</span><span class="nx">content</span><span class="p">,</span> <span class="nx">mods</span><span class="p">);</span>
+    <span class="p">}</span> <span class="k">catch</span> <span class="p">(</span><span class="nx">e</span><span class="p">)</span> <span class="p">{</span>
+      <span class="k">if</span> <span class="p">(</span><span class="nx">e</span> <span class="k">instanceof</span> <span class="nx">SyntaxError</span><span class="p">)</span> <span class="nx">raise</span><span class="p">(</span><span class="nx">start</span><span class="p">,</span> <span class="nx">e</span><span class="p">.</span><span class="nx">message</span><span class="p">);</span>
+      <span class="nx">raise</span><span class="p">(</span><span class="nx">e</span><span class="p">);</span>
+    <span class="p">}</span>
+    <span class="k">return</span> <span class="nx">finishToken</span><span class="p">(</span><span class="nx">_regexp</span><span class="p">,</span> <span class="nx">value</span><span class="p">);</span>
   <span class="p">}</span></pre></div>             </td>           </tr>                               <tr id="section-71">             <td class="docs">               <div class="pilwrap">                 <a class="pilcrow" href="#section-71">&#182;</a>               </div>               <p>Read an integer in the given radix. Return null if zero digits
 were read, the integer value otherwise. When <code>len</code> is given, this
 will return <code>null</code> unless the integer has exactly <code>len</code> digits.</p>             </td>             <td class="code">               <div class="highlight"><pre>  <span class="kd">function</span> <span class="nx">readInt</span><span class="p">(</span><span class="nx">radix</span><span class="p">,</span> <span class="nx">len</span><span class="p">)</span> <span class="p">{</span>