dx: descriptive permission errors (#3808)

Before:
```
▶ target/debug/deno https://deno.land/std/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/std/examples/echo_server.ts:4:23
```

```
▶ target/debug/deno --allow-read=/usr https://deno.land/std/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/std/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/std/examples/echo_server.ts:4:23
```

```
▶ target/debug/deno --allow-read=/usr https://deno.land/std/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)
```

1 files changed, 2 insertions, 2 deletions

diff --git a/std/fs/exists_test.ts b/std/fs/exists_test.ts
index b8c968a02..c7c7dc54e 100644
--- a/std/fs/exists_test.ts
+++ b/std/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