Updated JSON-LD API to raise exceptions on error. Closes #13.

Updated framing method to return null if there are no matches.

Author: msporny

spec/latest/index.html

@@ -1735,7 +1735,8 @@ <h3>JsonLdProcessor</h3>
     according to the steps in the
     <a href="#compaction-algorithm">Compaction Algorithm</a>. The
     <code>input</code> MUST be copied, compacted and returned if there are
-    no errors. If the compaction fails, <code>null</code> MUST be returned.
+    no errors. If the compaction fails, an appropirate exception MUST be
+    thrown.
   <dl class="parameters">
      <dt>object input</dt>
      <dd>The JSON-LD object to perform compaction on.</dd>
@@ -1771,10 +1772,9 @@ <h3>JsonLdProcessor</h3>
     using the <code>frame</code> according to the steps in the
     <a href="#framing-algorithm">Framing Algorithm</a>. The
     <code>input</code> is used to build the framed output and is returned if
-    there are no errors. Exceptions are thrown if there are errors.
-    <div class="issue">Define what the exceptions are. We need to specify
-    whether or not we want exceptions thrown, or errors returned to the
-    error callback?</div>
+    there are no errors. If there are no matches for the frame, 
+    <code>null</code> MUST be returned. Exceptions MUST be thrown if there are 
+    errors.
   <dl class="parameters">
      <dt>object input</dt>
      <dd>The JSON-LD object to perform framing on.</dd>