cmd/completion: use RunE instead of Run

General MR

Summary

Easy one here, this function calls Cobra functions that don't really do anything. Quickly convert it to RunE and wrap the error. Fix the tests.

Relevant logs/screenshots

bbctl completion fish | head -n 5

# fish completion for bbctl                                -*- shell-script -*-

function __bbctl_debug
    set -l file "$BASH_COMP_DEBUG_FILE"
    if test -n "$file"

Linked Issue

Closes #176 (closed)

Upgrade Notices

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