Add exception to Channel.

src/channel.js

 
 import { assert, NEED_TO_IMPLEMENT } from './utils.js';
 
+class ChnlStepConflictError extends Error {
+  constructor() {
+    super("Channel Step error");
+  }
+}
+
+
 class CpySchedule {
   first  = { pos : 0, size: 0 };
   second = { pos : 0, size: 0 };
@@ -406,7 +413,7 @@ export class Channel {
     }
 
     if (this.#writePhase != WRITE_PHASE.NO_STEP_WRITE)
-      throw new Error("Unable to call multiple writeStep1()");
+      throw new ChnlStepConflictError();
 
     this.#writePhase = WRITE_PHASE.WAIT_CALLER_WRITE;
     let schedule = this.#cpySchedule(size);
@@ -437,7 +444,7 @@ export class Channel {
 
   writeStep2() {
     if (this.#writePhase != WRITE_PHASE.WAIT_CALLER_WRITE)
-      throw new Error("Have to call writeStep1() first")
+      throw new ChnlStepConflictError();
     this.#writePointerUpdate(this.#writePhaseEndPos);
     this.#writePhase = WRITE_PHASE.NO_STEP_WRITE;
   }