Bad config error lacks complete field information

To reproduce

Mangle your entry in /etc/passwd so getpwuid will fail. Run env - target/debug/arti proxy

Expected output

An error message explaining which config keys (eg storage.keystore.path) couldn't be resolved and/or which env vars or whatever were missing and/or that getpwuid failed.

Actual output

target/debug/arti: error: Can't construct project directories to resolve a path element

NB that if #988 (closed) is fixed this particular repro may no longer be effective.