An album whose folder cannot be made no longer stops the backup (closes #197)

Each album's folder, links and JSON are written inside one try. A failure
prints `FAILED album <name>: <reason>`, lists the album's ID in the new
`failedAlbums` of the result and of `--json`, and makes `quak backup` exit 1;
the other albums and `failures.json` are still written.

An album whose folder would have another album's JSON name, ignoring case,
gets its ID. A name over 255 bytes (250 for a folder, which leaves room for
`.json`) gets the ID and is cut short between two characters by
`shortenToBytes` in `src/filename.ts`. A link's ID goes right before its
extension.

Apart from its call to `uniqueNames`, the album loop is only indented into
the try.

Model: opus-5-5

src/filename.ts

@@ -4,7 +4,8 @@
 // hands us, and quak does not trust the server. Any name taken from them and
 // used on disk goes through here, so it can only ever name one file inside the
 // directory the caller chose: never a path, never `..`, never hidden, never a
-// Windows device name.
+// Windows device name. Such a name can still be too long for a file name, and
+// `shortenToBytes` cuts one short.
 //
 // A path the user typed (`--out`, `outPath`) is not passed through here: the
 // caller is trusted, the server is not.
@@ -39,3 +40,20 @@ export const safeExtension = (title: string): string => {
 // `name` with its extension, if it has one, replaced by `ext` (".mov").
 export const withExtension = (name: string, ext: string): string =>
     name.slice(0, name.length - extname(name).length) + ext;
+
+// The longest file name, in bytes, that ext4 and most other file systems allow.
+export const MAX_NAME_BYTES = 255;
+
+// `name` cut short to at most `maxBytes` bytes of UTF-8, the encoding a file
+// name is written to disk in, between two characters, never inside one. A name
+// that fits comes back unchanged.
+export const shortenToBytes = (name: string, maxBytes: number): string => {
+    let bytes = 0;
+    let end = 0;
+    for (const character of name) {
+        bytes += Buffer.byteLength(character);
+        if (bytes > maxBytes) break;
+        end += character.length;
+    }
+    return name.slice(0, end);
+};