api/v0: improve error conditions

Return an HTTP 404 when no media is found and an HTTP 400 when some of
the requested tags aren't found.

lib/Pooru/API/V0/Controller/Media.pm

@@ -20,6 +20,10 @@ sub _list_with_tags ($self, $page, @tags)
 	my $paged_media_ids = $self->media_model->with_all_tags(@tags)
 	    ->page($page);
 	my @media_ids = map {$_->{media_id}} $paged_media_ids->fetchall->@*;
+	return $self->render(
+		json => {error => "No matching media found."},
+		status => 404,
+	) if @media_ids == 0;
 
 	return $self->render(json => {
 		media => $self->media_model->get(@media_ids)->page($page)
@@ -55,6 +59,11 @@ sub list ($self)
 	@tag_ids = map {$_->{id}} $self->tags_model
 	    ->get(display => [@tags])->fetchall->@* if @tags > 0;
 
+	return $self->render(
+		json => {error => "Some of the requested tags don't exist."},
+		status => 400,
+	) if @tag_ids != @tags && @tags > 0;
+
 	return @tags == 0 ?
 	    $self->_list_no_tags($page) :
 	    $self->_list_with_tags($page, @tag_ids);