Add mapError decoder method

Author: Elias Mulhall

src/decoder.ts

@@ -730,6 +730,14 @@ export class Decoder<A> {
   map = <B>(f: (value: A) => B): Decoder<B> =>
     new Decoder((json, context) => Result.map(f, this.decode(json, context)));
 
+  /**
+   * Construct a new decoder that applies a transformation to an error message
+   * on failure. If the decoder fails then `f` will be applied to the error. If
+   * it succeeds the value will propagated through.
+   */
+  mapError = (f: (error: DecoderError) => DecoderError): Decoder<A> =>
+    new Decoder((json, context) => Result.mapError(f, this.decode(json, context)));
+
   /**
    * Chain together a sequence of decoders. The first decoder will run, and
    * then the function will determine what decoder to run second. If the result