Clarified error format and handling.

Author: dblock

README.markdown

@@ -75,12 +75,22 @@ You can also return JSON formatted objects explicitly by raising error! and pass
 
 ## Exception Handling
 
-Grape can be told to rescue certain (or all) exceptions in your
-application and instead display them in text or json form. To do this,
-you simply use the `rescue_from` method inside your API declaration:
+Grape can be told to rescue all exceptions and instead return them in text or json formats.
 
     class Twitter::API < Grape::API
-      rescue_from ArgumentError, NotImplementedError # :all for all errors
+      rescue_from :all
+    end
+
+You can also rescue specific exceptions.
+
+    class Twitter::API < Grape::API
+      rescue_from ArgumentError, NotImplementedError
+    end
+
+The error format can be specified using `error_format`. Available formats are `:json` and `:txt` (default).
+
+    class Twitter::API < Grape::API
+      error_format :json
     end
 
 ## Note on Patches/Pull Requests