minor #18134 [Standards] Improve code standards about exception and error messages (alexandre-daubois)

This PR was merged into the 5.4 branch.

Discussion
----------

[Standards] Improve code standards about exception and error messages

After this one being merged symfony/symfony#49829, I think the coding standards about exception can be improved a bit 👍

Commits
-------

6d3fc80 [Standards] Improve code standards about exception and error messages

Author: javiereguiluz

contributing/code/standards.rst

@@ -179,6 +179,12 @@ Structure
 
 * Exception and error message strings must be concatenated using :phpfunction:`sprintf`;
 
+* Exception and error messages must not contain backticks, even when referring to a
+  technical element (such as a method name for example). Double quotes must be used
+  at all time;
+
+* Exception and error messages must start with a capital letter and finish with a dot ``.``;
+
 * Do not use ``else``, ``elseif``, ``break`` after ``if`` and ``case`` conditions
   which return or throw something;