"Invalid etag" on a stale local listing does not name the fix, and prints mtime as a hex float

[dmpetrov]

Description

Local-file etags derive from mtime, so regenerating or re-syncing source files invalidates a cached
listing and every read then fails:

FileNotFoundError: Invalid etag for file:///.../fixture/fold_shirt/1.mp4:
  expected 0x1.a9fab9eea1cbdp+30, got 0x1.a9fabd21cdfd9p+30

The behaviour is correct; the message is the problem. It does not mention
read_storage(..., update=True), which is the fix, and the hex float rendering of an mtime tells the
reader nothing.

Suggested fix

Name update=True in the message and render the timestamps as ISO dates.

Version Info

datachain 0.59.4.dev1+g1955f8e5b (editable, main @ 1955f8e5b)
Python 3.12.11, macOS 15 (Darwin 25.6.0)