Add error templates for 400 and label each item optional

Added documenttion on adding a _generic.400.html template which can be
used to display errors in FTS query syntax. It can include the chrome
supplied by page.html which makes navigating etc. easier.

Also labeled each section title in the current release optional as
none of them are required upgrades.

If we add a required step that will break a tracker if not done, it
should be labeled required and put at the top of the upgrading
section.

Author: rouilj

doc/upgrading.txt

@@ -35,8 +35,8 @@ Contents:
 Migrating from 2.1.0 to 2.x.y
 =============================
 
-Check compression settings
---------------------------
+Check compression settings (optional)
+-------------------------------------
 
 Read the `administration guide`_ section on 'Configuring Compression'.
 
@@ -50,15 +50,15 @@ merge any local comments from the tracker's ``config.ini`` into
 compression settings as you want. Then replace ``config.ini`` with the
 ``newconfig.ini`` file.
 
-Search added to user index page
--------------------------------
+Search added to user index page (optional)
+------------------------------------------
 
 A search form and count of number of hits has been added to the
 ``user.index.html`` template page in the classic template. You may
 want to merge the search form and footer into your template.
 
-Enhanced full-text search
--------------------------
+Enhanced full-text search (optional)
+------------------------------------
 
 SQLite's `FTS5 full-text search engine`_ is available. It should work
 with any recent sqlite3 installation. Any full text search field will
@@ -99,6 +99,21 @@ commands::
    delete from __words;
    delete from __textids;
 
+Adding error reporting templates (optional)
+-------------------------------------------
+
+Currently some internal errors result in a bare html page with an
+error message. The usual chrome supplied by page.html is not shown.
+For example query language syntax errors for full text search methods
+will display a bare HTML error page.
+
+If you add an `_generic.400.html` template to the html directory, you
+can display the error inside of the layout provided by the `page.html`
+template. This can make fixing the error and navigation easier. You
+can use the `_generic.404.html` template to create a
+`_generic.400.html` by modifying the title and body text. You can test
+the 400 template by appending `@template=400` to the url for the
+tracker.
 
 Migrating from 2.0.0 to 2.1.0
 =============================