Expose connection errors in HTTPFileSystem._exists

[btbest]

I can't see why blanket-catching all types of network and client-side issues would be a good thing to do here; the git history doesn't explain it, and test_http passes for me locally with this change. Encountering an error during session.get doesn't imply the file doesn't exist, so it generally seems wrong to return False.

I'm running into this issue because I want to implement retrying if the connection drops or times out. With the current implementation, I can't distinguish whether the server actually doesn't have the file vs. the user's mobile data reception being terrible.

[btbest]

Context: ilastik/ilastik#3019 (the last commit has the workaround without upstreaming this fix)

[martindurant]

Hm, in the case that the URL points to a non-existent server, exists(url) would now cause an exception rather than returning False.
It is possible that we could differentiate between "does not exist" and "other, maybe retriable error". But HTTP is tricky and generally non-standard, so it would need to consider cases like Gateway Error and Not Authenticated. In general, the right choice will depend on the remote you expect to interact with.