Footnotes: reject when failing to show footnote

Now `handle()` could return either undefined or a promise, which you can
catch to handle the case where footnote fails to show.

Also improve footnote extraction.

footnotes.js

@@ -32,11 +32,17 @@ const getReferencedType = el => {
 const isInline = 'a, span, sup, sub, em, strong, i, b, small, big'
 const extractFootnote = (doc, anchor) => {
     let el = anchor(doc)
+    const target = el
     while (el.matches(isInline)) {
         const parent = el.parentElement
         if (!parent) break
         el = parent
     }
+    if (el === doc.body) {
+        const sibling = target.nextElementSibling
+        if (sibling && !sibling.matches(isInline)) return sibling
+        throw new Error('Failed to extract footnote')
+    }
     return el
 }
 
@@ -44,7 +50,9 @@ export class FootnoteHandler extends EventTarget {
     detectFootnotes = true
     #showFragment(book, { index, anchor }, href) {
         const view = document.createElement('foliate-view')
+        return new Promise((resolve, reject) => {
             view.addEventListener('load', e => {
+                try {
                     const { doc } = e.detail
                     const el = anchor(doc)
                     const type = getReferencedType(el)
@@ -61,24 +69,30 @@ export class FootnoteHandler extends EventTarget {
                     }
                     const detail = { view, href, type, hidden, target: el }
                     this.dispatchEvent(new CustomEvent('render', { detail }))
+                    resolve()
+                } catch (e) {
+                    reject(e)
+                }
             })
             view.open(book)
                 .then(() => this.dispatchEvent(new CustomEvent('before-render', { detail: { view } })))
                 .then(() => view.goTo(index))
+                .catch(reject)
+        })
     }
     handle(book, e) {
         const { a, href } = e.detail
         const { yes, maybe } = isFootnoteReference(a)
         if (yes) {
             e.preventDefault()
-            Promise.resolve(book.resolveHref(href)).then(target =>
+            return Promise.resolve(book.resolveHref(href)).then(target =>
                 this.#showFragment(book, target, href))
         }
         else if (this.detectFootnotes && maybe()) {
             e.preventDefault()
-            Promise.resolve(book.resolveHref(href)).then(({ index, anchor }) => {
+            return Promise.resolve(book.resolveHref(href)).then(({ index, anchor }) => {
                 const target = { index, anchor: doc => extractFootnote(doc, anchor) }
-                this.#showFragment(book, target, href)
+                return this.#showFragment(book, target, href)
             })
         }
     }