I also noted that the suggested fix in the error message "spack clean --all" is not valid.

To reproduce:
spack create http://mvapich.cse.ohio-state.edu/download/mvapich2/mv2

rzmerl156<113>: spack create http://mvapich.cse.ohio-state.edu/download/mvapich2/mv2/mvapich2-1.9.tgz
==> Creating template for package mvapich2
==> Downloading...
==> Fetching http://mvapich.cse.ohio-state.edu/download/mvapich2/mv2/mvapich2-2.tgz

######################################################################################################### 100.0%

==> Warning: The contents of /g/g0/moody20/projects/spack.git/var/spack/stage/spack-stage-azl8Wu/mvapich2-2.tgz look like HTML.
The checksum will likely be bad. If it is, you can use
'spack clean --all' to remove the bad archive, then fix
your internet gateway issue and install again.
/bin/tar: This does not look like a tar archive

gzip: stdin: not in gzip format
/bin/tar: Child returned status 1
/bin/tar: Error is not recoverable: exiting now
==> Error: command '/bin/tar --exclude=//* -tf /g/g0/moody20/projects/spack.git/var/spack/stage/spack-stage-azl8Wu/mvapich2-2.tgz' returned error code 2