Better error message when starting urbit pointed at wrong directory

[zalberico]

See: #5875

When a user starts their pier from the wrong working directory (or specifying the wrong path to their pier) they get an error that isn't particularly helpful.

urbit 1.9
boot: home is /home/navluc-latmes
loom: mapped 2048MB
lite: arvo formula 11a9e7fe
lite: core 38d4ad4d
lite: final state 38d4ad4d
lmdb: meta read: dbi_open fail: MDB_NOTFOUND: No matching key/data pair found
disk: read meta: no version
pier: disk read meta fail

This is basically just ENOTAPIER. The first thing urbit does when it starts up is to read the metadata out of the event log, so if you point it at some random directory this is what you'll see. You're probably in the wrong working directory, or might have specified the wrong path to your pier (ship directory).

We should probably catch this more specifically and output something that says that an urbit ship is not found at the specified location.