cmd/cue/cmd: don't use os.Exit and fix error output
cmd/cue/cmd: don't use os.Exit and fix error output

Fixes #125

Change-Id: Iac275218e1eb6586a2a2f2345454082378571972
Reviewed-on: https://cue-review.googlesource.com/c/cue/+/3381
Reviewed-by: Marcel van Lohuizen <a22d082192f3ccf984644a4f482781f4c2d65180@golang.org>
Reviewed-by: Daniel Martí <29bf28b42a938ace12a5819da4ed4ba03d82f315@mvdan.cc>

cuelang/cue