Don't swallow errors on varUpdate()

Add MIError class for exceptions. Check error type on varUpdate() failure
and rethrow if it's not "Variable object not found".

src/backend/backend.ts

@@ -123,3 +123,38 @@ export class VariableObject {
 		return res;
 	}
 }
+
+// from https://gist.github.com/justmoon/15511f92e5216fa2624b#gistcomment-1928632
+export interface MIError extends Error {
+	readonly name: string;
+	readonly message: string;
+	readonly source: string;
+};
+export interface MIErrorConstructor {
+	new (message: string, source: string): MIError;
+	readonly prototype: MIError;
+}
+
+export const MIError: MIErrorConstructor = <any>class MIError {
+	readonly name: string;
+	readonly message: string;
+	readonly source: string;
+	public constructor(message: string, source: string) {
+		Object.defineProperty(this, 'name', {
+			get: () => (this.constructor as any).name,
+		});
+		Object.defineProperty(this, 'message', {
+			get: () => message,
+		});
+		Object.defineProperty(this, 'source', {
+			get: () => source,
+		});
+		Error.captureStackTrace(this, this.constructor);
+	}
+
+	public toString() {
+		return `${this.message} (from ${this.source})`;
+	}
+};
+Object.setPrototypeOf(MIError as any, Object.create(Error.prototype));
+MIError.prototype.constructor = MIError;

src/backend/mi2/mi2.ts

@@ -1,4 +1,4 @@
-import { Breakpoint, IBackend, Stack, SSHArguments, Variable, VariableObject } from "../backend"
+import { Breakpoint, IBackend, Stack, SSHArguments, Variable, VariableObject, MIError } from "../backend"
 import * as ChildProcess from "child_process"
 import { EventEmitter } from "events"
 import { parseMI, MINode } from '../mi_parse';
@@ -729,11 +729,11 @@ export class MI2 extends EventEmitter implements IBackend {
 			this.handlers[sel] = (node: MINode) => {
 				if (node && node.resultRecords && node.resultRecords.resultClass === "error") {
 					if (suppressFailure) {
-						this.log("stderr", "WARNING: Error executing command '" + command + "'");
+						this.log("stderr", `WARNING: Error executing command '${command}'`);
 						resolve(node);
 					}
 					else
-						reject((node.result("msg") || "Internal error") + " (from " + command + ")");
+						reject(new MIError(node.result("msg") || "Internal error", command));
 				}
 				else
 					resolve(node);