Add notFound suppor (& test) for assign REST (#1902)

Signed-off-by: Marinov Avgustin <Avgustin.Marinov@bosch.com>

hawkbit-rest/hawkbit-mgmt-api/src/main/java/org/eclipse/hawkbit/mgmt/rest/api/MgmtTargetTagRestApi.java

@@ -335,6 +335,9 @@ public interface MgmtTargetTagRestApi {
             @ApiResponse(responseCode = "401", description = "The request requires user authentication."),
             @ApiResponse(responseCode = "403", description = "Insufficient permissions, entity is not allowed to be " +
                     "changed (i.e. read-only) or data volume restriction applies."),
+            @ApiResponse(responseCode = "404", description = "Not Found - e.g. target tag not found. If targets not found - " +
+                    "the body contains a list of ",
+                    content = @Content(mediaType = "application/json", schema = @Schema(implementation = ExceptionInfo.class))),
             @ApiResponse(responseCode = "405", description = "The http request method is not allowed on the resource."),
             @ApiResponse(responseCode = "406", description = "In case accept header is specified and not application/json."),
             @ApiResponse(responseCode = "409", description = "E.g. in case an entity is created or modified by another " +