When you click a link to deleted (or just nonexistent) revision on the web, [[https://en.wikipedia.org/wiki/MediaWiki:Difference-missing-revision|MediaWiki:Difference-missing-revision]] is shown which explains what happened.
* Example: https://en.wikipedia.org/wiki/Special:Diff/898964501898964501
But when you click the same link in mobile view, it shows unhelpful plain error message:
`Bad revision. The ID for the revision you requested does not exist.`
* https://en.m.wikipedia.org/wiki/Special:MobileDiff/898964501898964501
###Developer notes
The problem is located in [[ https://github.com/wikimedia/mediawiki-extensions-MobileFrontend/blob/f4074ae9329b68ad3bc2067731d93bda01cfe6d3/includes/specials/SpecialMobileDiff.php#L100 | SpecialMobileDiff::executeWhenAvailable() ]] function. When $rev is null (no revision found), instead of normal flow system calls
`wfHttpError( 404, title, text );` and returns early. wfHttpError function returns the error in oldchool/ugly way without any layout (as it says, it's simple http errror.
Code:
```
public function executeWhenAvailable( $par ) {
[...]
$revisions = $this->getRevisionsToCompare( explode( '...', $par, 2 ) );
list( $prev, $rev ) = $revisions;
if ( $rev === null ) {
$this->executeBadQuery();
return false;
}
[...]
}
/**
* Generate a 404 Error message, that revisions can not be found
*/
public function executeBadQuery() {
wfHttpError( 404, $this->msg( 'mobile-frontend-diffview-404-title' )->text(),
$this->msg( 'mobile-frontend-diffview-404-desc' )->text() );
}
```
Instead please use `$this->showPageNotFound()`. TheSpecialMobileDiff should override `errorNotFoundDescriptionMsg` and `errorNotFoundTitleMsg` properties.
####Acceptance criteria
[x] When $revision is not found system outputs nice error page
[x] wfHttpError() is not used
### QA
Visit https://en.m.wikipedia.beta.wmflabs.org/wiki/Special:MobileDiff/898964501898964501
This should show a friendly error message in the Minerva skin. It should **not** show a blank white page with the text "The ID for the revision you requested does not exist."