improve UI: export command fails: No such file or directory

[whilei]

~/Library/EthereumClassic  ⟠ which emerald
/usr/local/bin/emerald

~/Library/EthereumClassic  ⟠ emerald -V
v0.19.0

~/Library/EthereumClassic  ⟠ emerald new --chain mainnet
! Warning: passphrase can't be restored. Don't forget it !
Enter passphrase:

Created new account: 0x2c1ed20222ef60eb4d88cc54c18319ec7fbf547d

~/Library/EthereumClassic  ⟠ emerald list
ADDRESS                                       NAME
0x2c1ed20222ef60eb4d88cc54c18319ec7fbf547d
0xc1243818f7b39454310b3b4262a2a83882e0150f    testes

~/Library/EthereumClassic  ⟠ emerald export 0x2c1ed20222ef60eb4d88cc54c18319ec7fbf547d ./add.json
[ERROR]	Command execution error: No such file or directory (os error 2)
~/Library/EthereumClassic  ⟠ emerald export 0x2c1ed20222ef60eb4d88cc54c18319ec7fbf547d $pwd/add.json
[ERROR]	Command execution error: No such file or directory (os error 2)
~/Library/EthereumClassic  ⟠ emerald export --chain mainnet 0x2c1ed20222ef60eb4d88cc54c18319ec7fbf547d ./add.json
[ERROR]	Command execution error: No such file or directory (os error 2)

~/Library/EthereumClassic  ⟠ emerald --help | grep export
  emerald export    [--chain=<chain>]  ([-a | --all] | <address>) <path>

[whilei]

Aha -- solved.

~/Library/EthereumClassic  ⟠ emerald export --chain mainnet 0x2c1ed20222ef60eb4d88cc54c18319ec7fbf547d .

Maybe could add more useful error feedback -- like: <path> must be a path to an existing directory or something

[whilei]

Also, when it does successfully export, it would be nice to get the path/name of the file to stdout, so as a human or program would know which file was generated.