dx: descriptive permission errors (denoland/deno#3808)

Before:
```
▶ target/debug/deno https://deno.land/examples/echo_server.ts
error: Uncaught PermissionDenied: run again with the --allow-net flag
► $deno$/dispatch_json.ts:40:11
    at DenoError ($deno$/errors.ts:20:5)
    at unwrapResponse ($deno$/dispatch_json.ts:40:11)
    at sendSync ($deno$/dispatch_json.ts:67:10)
    at listen ($deno$/net.ts:170:15)
    at https://deno.land/examples/echo_server.ts:4:23
```

```
▶ target/debug/deno --allow-read=/usr https://deno.land/examples/cat.ts /etc/passwd
error: Uncaught PermissionDenied: run again with the --allow-read flag
► $deno$/dispatch_json.ts:40:11
    at DenoError ($deno$/errors.ts:20:5)
    at unwrapResponse ($deno$/dispatch_json.ts:40:11)
    at sendAsync ($deno$/dispatch_json.ts:91:10)
```

After:
```
▶ target/debug/deno https://deno.land/examples/echo_server.ts
error: Uncaught PermissionDenied: network access to "0.0.0.0:8080", run again with the --allow-net flag
► $deno$/dispatch_json.ts:40:11
    at DenoError ($deno$/errors.ts:20:5)
    at unwrapResponse ($deno$/dispatch_json.ts:40:11)
    at sendSync ($deno$/dispatch_json.ts:67:10)
    at listen ($deno$/net.ts:170:15)
    at https://deno.land/examples/echo_server.ts:4:23
```

```
▶ target/debug/deno --allow-read=/usr https://deno.land/examples/cat.ts /etc/passwd
error: Uncaught PermissionDenied: read access to "/etc/passwd", run again with the --allow-read flag
► $deno$/dispatch_json.ts:40:11
    at DenoError ($deno$/errors.ts:20:5)
    at unwrapResponse ($deno$/dispatch_json.ts:40:11)
    at sendAsync ($deno$/dispatch_json.ts:91:10)
```

fs/empty_dir_test.ts

@@ -2,6 +2,7 @@
 import { test } from "../testing/mod.ts";
 import {
   assertEquals,
+  assertStrContains,
   assertThrows,
   assertThrowsAsync
 } from "../testing/asserts.ts";
@@ -227,7 +228,7 @@ test(async function emptyDirPermission(): Promise<void> {
 
       const output = await Deno.readAll(stdout);
 
-      assertEquals(new TextDecoder().decode(output), s.output);
+      assertStrContains(new TextDecoder().decode(output), s.output);
     }
   } catch (err) {
     await Deno.remove(testfolder, { recursive: true });

fs/exists_test.ts

@@ -1,6 +1,6 @@
 // Copyright 2018-2020 the Deno authors. All rights reserved. MIT license.
 import { test } from "../testing/mod.ts";
-import { assertEquals } from "../testing/asserts.ts";
+import { assertEquals, assertStrContains } from "../testing/asserts.ts";
 import * as path from "../path/mod.ts";
 import { exists, existsSync } from "./exists.ts";
 
@@ -134,7 +134,7 @@ test(async function existsPermission(): Promise<void> {
 
     const output = await Deno.readAll(stdout);
 
-    assertEquals(new TextDecoder().decode(output), s.output);
+    assertStrContains(new TextDecoder().decode(output), s.output);
   }
 
   // done

http/file_server_test.ts

@@ -1,6 +1,6 @@
 // Copyright 2018-2020 the Deno authors. All rights reserved. MIT license.
 import { test } from "../testing/mod.ts";
-import { assert, assertEquals } from "../testing/asserts.ts";
+import { assert, assertEquals, assertStrContains } from "../testing/asserts.ts";
 import { BufReader } from "../io/bufio.ts";
 import { TextProtoReader } from "../textproto/mod.ts";
 
@@ -109,8 +109,8 @@ test(async function servePermissionDenied(): Promise<void> {
 
   try {
     await fetch("http://localhost:4500/");
-    assertEquals(
-      await errReader.readLine(),
+    assertStrContains(
+      (await errReader.readLine()) as string,
       "run again with the --allow-read flag"
     );
   } finally {

manual.md

@@ -229,7 +229,7 @@ When this program is started, it throws PermissionDenied error.
 
 ```shell
 $ deno https://deno.land/std/examples/echo_server.ts
-error: Uncaught PermissionDenied: run again with the --allow-net flag
+error: Uncaught PermissionDenied: network access to "0.0.0.0:8080", run again with the --allow-net flag
 ► $deno$/dispatch_json.ts:40:11
     at DenoError ($deno$/errors.ts:20:5)
     ...
@@ -329,7 +329,7 @@ This is an example to restrict file system access by whitelist.
 
 ```shell
 $ deno --allow-read=/usr https://deno.land/std/examples/cat.ts /etc/passwd
-error: Uncaught PermissionDenied: run again with the --allow-read flag
+error: Uncaught PermissionDenied: read access to "/etc/passwd", run again with the --allow-read flag
 ► $deno$/dispatch_json.ts:40:11
     at DenoError ($deno$/errors.ts:20:5)
     ...