unit already exists error should not print the usage display

[motatoes]

$ taco unit create otaco-digger-dev
[CREDS DEBUG] Primary: terraform token
Error: failed to create unit: HTTP 409: Unit already exists
Usage:
  taco unit create <unit-id> [flags]

Flags:
  -h, --help   help for create

Global Flags:
      --server string   OpenTaco server URL (default "http://localhost:8080")
  -v, --verbose         Enable verbose output

failed to create unit: HTTP 409: Unit already exists

[MrKeiKun]

after checking, i saw in digger/taco/cmd/taco/commands/unit.go. it seems like:

1. User runs: taco unit create existing-unit
2. Server responds: HTTP 409 "Unit already exists"
3. SDK parses this: Creates error message "HTTP 409: Unit already exists"
4. CLI returns error: The unitCreateCmd function returns this error
5. Cobra displays usage: Since RunE returned an error, Cobra automatically shows the usage information (sad part)

easy fix, gonna send the fix here ASAP for more hacktoberfest. 💟