Fix username validation and exception handling

1. Refactor string validation classes
2. Rename string validation assets for length
3. Improve exception handling when server is not able to create requested server

@@ -0,0 +7,4 @@
+class FieldCubitFactory {
+  FieldCubitFactory(this.context);
+
+  /* A common user login field.

@@ -0,0 +35,4 @@
+    );
+  }
+
+  /* A common user password field.

@@ -12,2 +13,4 @@
   }
 }
+
+// String must be equal to [length]