Refactor error classes

All runner errors are now in a single file. The not found error
has been splitted into an error for runner not found and for
environment not found.

app/errors/runner/error.rb

@@ -1,3 +1,21 @@
 # frozen_string_literal: true
 
-class Runner::Error < ApplicationError; end
+class Runner
+  class Error < ApplicationError
+    class BadRequest < Error; end
+
+    class EnvironmentNotFound < Error; end
+
+    class ExecutionTimeout < Error; end
+
+    class InternalServerError < Error; end
+
+    class NotAvailable < Error; end
+
+    class Unauthorized < Error; end
+
+    class RunnerNotFound < Error; end
+
+    class Unknown < Error; end
+  end
+end