actool position-sensitive arguments and error messages confusing

[vcaputo]

e.g.

 # actool build imgs/bash imgs/bash.aci
 build: Target file exists (try --overwrite)
 # actool build imgs/bash imgs/bash.aci --overwrite
 build: Must provide directory and output file
 # actool build --overwrite imgs/bash imgs/bash.aci
 #