Make cleaner error on Dockerfile build errors
Make cleaner error on Dockerfile build errors

This is a fix for https://bugzilla.redhat.com/show_bug.cgi?id=1695622

If the build process had an error during the Dockerfile processing, the
error message was very verbose.  This change moves the original
message to be shown during a debug session and shortens the
errors to the heart of the problem.

Testing:
-------------------

```
OLD:
STEP 1: FROM alpine
STEP 2: ADD reallybadfile /tmp
error building at step {Env:[PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin] Command:add Args:[reallybadfile /tmp] Flags:[] Attrs:map[] Message:ADD reallybadfile /tmp Original:ADD reallybadfile /tmp}: no files found matching "/root/tsweeney/workspaces/buildah/src/github.com/containers/buildah/reallybadfile": no such file or directory
[root@localhost buildah]# pwd
/root/tsweeney/workspaces/buildah/src/github.com/containers/buildah

NEW:
STEP 1: FROM alpine
STEP 2: ADD reallybadfile /tmp
error building at STEP "ADD reallybadfile /tmp": no files found matching "/root/tsweeney/workspaces/buildah/src/github.com/containers/buildah/reallybadfile": no such file or directory

STEP 2: ADD reallybadfile /tmp
DEBU[0000] ADD []string(nil), imagebuilder.Copy{FromFS:false, From:"", Src:[]string{"reallybadfile"}, Dest:"/tmp", Download:true, Chown:""}
DEBU[0000] error building at step {Env:[PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin] Command:add Args:[reallybadfile /tmp] Flags:[] Attrs:map[] Message:ADD reallybadfile /tmp Original:ADD reallybadfile /tmp}: no files found matching "/root/tsweeney/workspaces/buildah/src/github.com/containers/buildah/reallybadfile": no such file or directory
error building at STEP "ADD reallybadfile /tmp": no files found matching "/root/tsweeney/workspaces/buildah/src/github.com/containers/buildah/reallybadfile": no such file or directory

```
Signed-off-by: TomSweeneyRedHat <tsweeney@redhat.com>

Closes: #1482
Approved by: rhatdan

projectatomic/buildah,nalind/buildah-1,nalind/buildah-1,TomSweeneyRedHat/buildah,nalind/buildah-1,nalind/buildah,TomSweeneyRedHat/buildah,nalind/buildah,rhatdan/buildah,projectatomic/buildah,rhatdan/buildah,TomSweeneyRedHat/buildah,projectatomic/buildah,rhatdan/buildah