filestore: use filepath pkg, track close err, lints

[jsign]

Some minors while reading this pkg, most relevant:

• use filepath to rely on stdlib
• since Store codeflow is simple enough, we can avoid defer to go the extra mile to see if closing fails.

[mishmosh]

Thank you for this! We will review in the new year.

[hannahhoward]

LGTM by me.

Please rebase so we can merge

[jsign]

Done.
Seems that in the meantime some other commit leveraged the filepath package, so that isn't a new change anymore. (So updated the commit msg).