vg_read: sometimes ignore read errors

vg_read: sometimes ignore read errors

Running "vgremove -f VG & pvs" results in the pvs
command reporting that the VG is not found or is
inconsistent.  If the VG is gone or being removed,
the pvs command should just skip it and not print
errors about it.

"Not found" is because the pvs command created the
list of VGs to process, including VG, then vgremove
removed the VG, then the pvs command came to to read
the VG to process it and did not find it.

An "inconsistent" error could be reported if vgremove
had only partially completed removing VG when pvs did
vg_read on the VG to process it, causing pvs to find
the VG in a partially-removed state.

This fix adds a flag that pvs uses to ignore a VG
that can't be read or is inconsistent.