Make `processUrlTemplate` fail if script origin is unknown

`document.currentScript` is supported by all browsers that we support.
If it returns `null` this indicates that it was called in the wrong
context and so fail loudly to make this obvious.

src/boot/test/url-template-test.js

@@ -38,12 +38,11 @@ describe('processUrlTemplate', () => {
         .returns([{ src: 'http://test-host:3001/script.js' }]);
     });
 
-    it('falls back to using origin info from the last <script> tag in the document', () => {
-      const url = processUrlTemplate(
-        '{current_scheme}://{current_host}:2000/style.css',
-        fakeDocument
-      );
-      assert.equal(url, 'http://test-host:2000/style.css');
+    it('throws if script origin cannot be determined', () => {
+      assert.throws(() => {
+        const template = '{current_scheme}://{current_host}:2000/style.css';
+        processUrlTemplate(template, fakeDocument);
+      }, 'Could not process URL template because script origin is unknown');
     });
 
     it('does not try to determine the origin if there are no URL template params', () => {

src/boot/url-template.js

@@ -14,19 +14,12 @@ function extractOrigin(url) {
 }
 
 function currentScriptOrigin(document_ = document) {
-  try {
-    let scriptEl = /** @type {HTMLScriptElement} */ (document_.currentScript);
-
+  const scriptEl = /** @type {HTMLScriptElement|null} */ (document_.currentScript);
   if (!scriptEl) {
-      // Fallback for IE 11.
-      const scripts = document_.querySelectorAll('script');
-      scriptEl = scripts[scripts.length - 1];
-    }
-
-    return extractOrigin(scriptEl.src);
-  } catch (err) {
+    // Function was called outside of initial script execution.
     return null;
   }
+  return extractOrigin(scriptEl.src);
 }
 
 /**
@@ -52,6 +45,10 @@ export default function processUrlTemplate(url, document_ = document) {
   if (origin) {
     url = url.replace('{current_host}', origin.hostname);
     url = url.replace('{current_scheme}', origin.protocol);
+  } else {
+    throw new Error(
+      'Could not process URL template because script origin is unknown'
+    );
   }
 
   return url;