Failing useFile hook with Blockstack 21

[njordhov]

When using version 21 of the Blockstack SDK, the useFile hook fails if the file does not exist. This will present with an error in the log and no setter returned by the hook to create the file.

The reason is a change in the SDK: getFile now throws error on 404 instead of returning null.

[friedger]

fixed in 0.6.9 and higher