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/user/email_address.go

@@ -40,6 +40,10 @@ func (err ErrEmailCharIsNotSupported) Error() string {
 	return fmt.Sprintf("e-mail address contains unsupported character [email: %s]", err.Email)
 }
 
+func (err ErrEmailCharIsNotSupported) Unwrap() error {
+	return util.ErrInvalidArgument
+}
+
 // ErrEmailInvalid represents an error where the email address does not comply with RFC 5322
 // or has a leading '-' character
 type ErrEmailInvalid struct {
@@ -56,6 +60,10 @@ func (err ErrEmailInvalid) Error() string {
 	return fmt.Sprintf("e-mail invalid [email: %s]", err.Email)
 }
 
+func (err ErrEmailInvalid) Unwrap() error {
+	return util.ErrInvalidArgument
+}
+
 // ErrEmailAlreadyUsed represents a "EmailAlreadyUsed" kind of error.
 type ErrEmailAlreadyUsed struct {
 	Email string
@@ -71,6 +79,10 @@ func (err ErrEmailAlreadyUsed) Error() string {
 	return fmt.Sprintf("e-mail already in use [email: %s]", err.Email)
 }
 
+func (err ErrEmailAlreadyUsed) Unwrap() error {
+	return util.ErrAlreadyExist
+}
+
 // ErrEmailAddressNotExist email address not exist
 type ErrEmailAddressNotExist struct {
 	Email string
@@ -86,6 +98,10 @@ func (err ErrEmailAddressNotExist) Error() string {
 	return fmt.Sprintf("Email address does not exist [email: %s]", err.Email)
 }
 
+func (err ErrEmailAddressNotExist) Unwrap() error {
+	return util.ErrNotExist
+}
+
 // ErrPrimaryEmailCannotDelete primary email address cannot be deleted
 type ErrPrimaryEmailCannotDelete struct {
 	Email string
@@ -101,6 +117,10 @@ func (err ErrPrimaryEmailCannotDelete) Error() string {
 	return fmt.Sprintf("Primary email address cannot be deleted [email: %s]", err.Email)
 }
 
+func (err ErrPrimaryEmailCannotDelete) Unwrap() error {
+	return util.ErrInvalidArgument
+}
+
 // EmailAddress is the list of all email addresses of a user. It also contains the
 // primary email address which is saved in user table.
 type EmailAddress struct {