update README with info about param coercion failures

Author: Matt Zukowski

README.md

@@ -408,7 +408,7 @@ desc 'Returns your public timeline.' do
             description: 'Not really needed',
             required: false
           }
-          
+
 end
 get :public_timeline do
   Status.limit(20)
@@ -751,7 +751,9 @@ Aside from the default set of supported types listed above, any class can be
 used as a type so long as an explicit coercion method is supplied. If the type
 implements a class-level `parse` method, Grape will use it automatically.
 This method must take one string argument and return an instance of the correct
-type, or raise an exception to indicate the value was invalid. E.g.,
+type. An exception raised inside the `parse` method will be reported as a validation
+failure with a generic error message. To report a custom error message, return an
+`InvalidValue` initialized with the custom message. E.g.,
 
 ```ruby
 class Color
@@ -761,8 +763,11 @@ class Color
   end
 
   def self.parse(value)
-    fail 'Invalid color' unless %w(blue red green).include?(value)
-    new(value)
+    if  %w(blue red green).include?(value)
+      new(value)
+    else
+      Grape::Validations::Types::InvalidValue.new "is not a valid color"
+    end
   end
 end