Make every not exist error unwrappable to a fs.ErrNotExist (#20891)

A lot of our code is repeatedly testing if individual errors are
specific types of Not Exist errors. This is repetitative and unnecesary.
`Unwrap() error` provides a common way of labelling an error as a
NotExist error and we can/should use this.

This PR has chosen to use the common `io/fs` errors e.g.
`fs.ErrNotExist` for our errors. This is in some ways not completely
correct as these are not filesystem errors but it seems like a
reasonable thing to do and would allow us to simplify a lot of our code
to `errors.Is(err, fs.ErrNotExist)` instead of
`package.IsErr...NotExist(err)`

I am open to suggestions to use a different base error - perhaps
`models/db.ErrNotExist` if that would be felt to be better.


Signed-off-by: Andrew Thornton <art27@cantab.net>
Co-authored-by: delvh <dev.lh@web.de>

models/db/engine.go

@@ -225,7 +225,7 @@ func NamesToBean(names ...string) ([]interface{}, error) {
 	for _, name := range names {
 		bean, ok := beanMap[strings.ToLower(strings.TrimSpace(name))]
 		if !ok {
-			return nil, fmt.Errorf("No table found that matches: %s", name)
+			return nil, fmt.Errorf("no table found that matches: %s", name)
 		}
 		if !gotBean[bean] {
 			beans = append(beans, bean)