Fix correct exit code on remote/local task error

[alajmo]

What's Changed

Concerns #23.

Changes:

• If a task fails, then return the last exit code from said task iff --any-errors-fatal=true, otherwise the exit code will be 0
• If a host is unreachable and --ignore-unreachable=false, return error code 4

There's some additional work that needs to be done before the v1.0.0 release, such as:

• What error codes to use for:
  • Syntax issues
  • Unreachable
  • Partial error
    • situations where hosts return different exit codes (one returns exit code 4, one returns exit code 9, one returns exit code 0 for instance, what should sake show?)

but I'll do the work in a separate PR when the feature set is complete (so as not to redo work).