vfs: Ensure file not found message is intelligable

tests/vfs-tests.js

@@ -17,6 +17,20 @@ function abstractVFSBehavior( VFS, name ) {
 				const sut = new VFS();
 				expect(await sut.exists("nonexistent-file")).to.eq(false);
 			});
+
+			it("raises a reasonable error the file does not exist", async function () {
+				const exampleFileName = "non-existent-file";
+				let raised = false;
+				try {
+					const sut = new VFS();
+					await sut.asBytes(exampleFileName);
+				}catch(e){
+					raised = true;
+					expect(e.message).to.include(exampleFileName);
+				}finally {
+					expect(raised).to.eq(true);
+				}
+			})
 		});
 
 		describe("When creating a file as bytes", function(){

vfs.js

@@ -20,20 +20,25 @@ class InMemoryVFS {
 		return !!this.files[file];
 	}
 
+	async _expectFile( file ){
+		if( !this.files[file]){
+			throw new Error("No such file " + file);
+		}
+	}
+
 	async unlink( file ){
 		if( this.files[file] ){
 			delete this.files[file];
 		}
 	}
 
 	async createReadableStream( file ){
-		if( !(await this.exists(file)) ){
-			throw new Error("No such file "+ file);
-		}
+		await this._expectFile(file);
 		return new MemoryReadable(this.files[file]);
 	}
 
 	async asBytes( file ){
+		await this._expectFile(file);
 		return Buffer.from(this.files[file]);
 	}