Create error_mapping.rst.inc

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | yes
| Applies to    | 2.1+
| Fixed tickets | ~

see http://symfony.com/blog/form-goodness-in-symfony-2-1#error-mapping-fu
include references to error_mapping in field inheritance
align table pipes
wrap lines
move versionadded to top of the file
Merge pull request #2694 from docteurklein/patch-2

add error_mapping form option documentation