chore: Better error messages

[XavierChanth]

- What I did

Due to some changes in the way arg parsing is done, the error messages for a bad parse is unpleasant, especially if you just want to see usage (this is what you get if you type sshnp, yuck):

Version : 3.4.0
-k, --key-file             Sending atSign's atKeys file if not in ~/.atsign/keys/
-f, --from (mandatory)     Sending (a.k.a. client) atSign
-t, --to (mandatory)       Receiving device atSign
-d, --device               Receiving device name
                           (defaults to "default")
-h, --host (mandatory)     atSign of sshrvd daemon or FQDN/IP address to connect back to
-p, --port                 TCP port to connect back to (only required if --host specified a FQDN/IP)
                           (defaults to "22")
-l, --local-port           Reverse ssh port to listen on, on your local machine, by sshnp default finds a spare port
                           (defaults to "0")
-s, --ssh-public-key       Public key file from ~/.ssh to be appended to authorized_hosts on the remote device
                           (defaults to "false")
-o, --local-ssh-options    Add these commands to the local ssh command
-v, --[no-]verbose         More logging
-r, --[no-]rsa             Use RSA 4096 keys rather than the default ED25519 keys
-u, --remote-user-name     username to use in the ssh session on the remote host
    --config-file          Read args from a config file
                           Mandatory args are not required if already supplied in the config file
Null check operator used on a null value
Unhandled exception:
Null check operator used on a null value
#0      new SSHNPParams.fromPartial (package:sshnoports/sshnp/sshnp_params.dart:53)
#1      SSHNPImpl.fromCommandLineArgs (package:sshnoports/sshnp/sshnp_impl.dart:189)
#2      SSHNP.fromCommandLineArgs (package:sshnoports/sshnp/sshnp.dart:149)
#3      main (file:///Users/chant/src/af/sshnoports/packages/sshnoports/bin/sshnp.dart:14)
#4      _delayEntrypointInvocation.<anonymous closure> (dart:isolate-patch/isolate_patch.dart:294)
#5      _RawReceivePort._handleMessage (dart:isolate-patch/isolate_patch.dart:189)

Change: Moved creation of SSHNP / SSHNPD/ SSHRVD class into the try block, and added a separate on catch:

} on ArgumentError catch (_) {
    exit(1);
  }

Net change is cleaner, easier to read ArgumentError messages when printing usage:

Version : 3.4.0
-k, --key-file             Sending atSign's atKeys file if not in ~/.atsign/keys/
-f, --from (mandatory)     Sending (a.k.a. client) atSign
-t, --to (mandatory)       Receiving device atSign
-d, --device               Receiving device name
                           (defaults to "default")
-h, --host (mandatory)     atSign of sshrvd daemon or FQDN/IP address to connect back to
-p, --port                 TCP port to connect back to (only required if --host specified a FQDN/IP)
                           (defaults to "22")
-l, --local-port           Reverse ssh port to listen on, on your local machine, by sshnp default finds a spare port
                           (defaults to "0")
-s, --ssh-public-key       Public key file from ~/.ssh to be appended to authorized_hosts on the remote device
                           (defaults to "false")
-o, --local-ssh-options    Add these commands to the local ssh command
-v, --[no-]verbose         More logging
-r, --[no-]rsa             Use RSA 4096 keys rather than the default ED25519 keys
-u, --remote-user-name     username to use in the ssh session on the remote host
    --config-file          Read args from a config file
                           Mandatory args are not required if already supplied in the config file

Invalid argument(s): Option from is mandatory.

- How I did it

- How to verify it

- Description for the changelog
chore: Better error messages