fix: only check fragments inside files

[kemingy]

• related to Checking fragments in local files results in 'Cannot read input content from' errors and eventually 'Too many open files' #1709

What's in this PR:

• only check fragments for files
• better err type
• more info in err msg

BTW, I'm not sure if the Client.check_fragment is used by any caller. cc @mre

[mre]

You're right, I think we can clean that up. Nice catch!
(Guess it was still in there because it belongs to the public API, but we don't have a stable API yet and its better to clean it up before we stabilize.)

[mre]

Thanks for the quick turnaround on this one! Can you post a comment in the linked issue?

[kemingy]

Thanks for the quick turnaround on this one! Can you post a comment in the linked issue?

Sure.