Throw a few more errors

Author: noamr

fetch.bs

@@ -8646,6 +8646,14 @@ partial interface mixin WindowOrWorkerGlobalScope {
 method steps are:
 
 <ol>
+ <li>
+  <p>If the user-agent has determined that deferred fetching is not allowed in this context, then
+  throw a {{NotAllowedError}}.
+
+  <p class=note>User-agents can, for example, decide that deferred fetching is not allowed for
+  subframes or gated behind some permission.
+ </li>
+
  <li><p>Let <var>requestObject</var> be the result of invoking the initial value of {{Request}} as
  constructor with <var>input</var> and <var>init</var> as arguments. This may throw an exception.
 
@@ -8654,6 +8662,12 @@ method steps are:
 
  <li><p>Let <var>request</var> be <var>requestObject</var>'s <a for=Request>request</a>.
 
+ <li><p>If <var>request</var>'s <a for=request>URL</a>'s <a for=url>scheme</a> is not an
+ <a>HTTP(S) scheme</a> then throw a {{TypeError}}.
+
+ <li><p>If <var>request</var>'s <a for=request>URL</a> is not a
+ [=potentially trustworthy url=], then throw a {{SecurityError}}.
+
  <li><p>Let <var>backgroundTimeout</var> be null.
 
  <li><p>If <var>init</var> is given and <var>init</var>["<code>backgroundTimeout</code>"]