Enhance error on unsafe symbolic link targets
Enhance error on unsafe symbolic link targets

If a COPY target was a symbolic link that resolved at a directory
above the context directory, securejoin gave an error that wasn't clear
to the end user (or this developer):

```
STEP 4: COPY README.md /
error building at STEP "COPY README.md /": secure join: too many levels of symbolic links
ERRO exit status 1
```

This change updates that error message to hopefully clear the issue up
for the user.  At the very least it will be easier to find in the code:

```
STEP 4: COPY README.md /
error building at STEP "COPY README.md /": forbidden path for "README.md", it is outside of the build context "/root/tsweeney/workspaces/containers/toolbox/images/fedora/f31": secure join: too many levels of symbolic links
```
Addressed #1952

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

Closes: #1970
Approved by: @rhatdan

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