chore: restore and deprecate MdError class

Author: crisbeto

src/lib/core/core.ts

@@ -76,6 +76,9 @@ export {MdLineModule, MdLine, MdLineSetter} from './line/line';
 // Style
 export * from './style/index';
 
+// Error
+export {MdError} from './errors/error';
+
 // Misc
 export {ComponentType} from './overlay/generic-component-type';
 

src/lib/core/errors/error.ts

@@ -0,0 +1,13 @@
+// TODO(kara): Revisit why error messages are not being properly set.
+
+/**
+ * Wrapper around Error that sets the error message.
+ * @docs-private
+ * @deprecated
+ */
+export class MdError extends Error {
+  constructor(value: string) {
+    super();
+    this.message = value;
+  }
+}