"pkg install" error messages for bad filenames or missing -forge option

[Andrew Janke]

Hi, Octave maintainers,

I got some wonky error messages when I forget the -forge option to "pkg 
install" or pass it a bad file name.

octave:1> pkg install foobar
warning: unpack: unrecognized FILETYPE <>
dirlist(3): out of bound 2
error: called from
    install at line 102 column 19
    pkg at line 437 column 9
octave:1> pkg install http://some-bad-url/foo/bar/baz.tgz
gzip: 
/var/folders/_4/9mx5ryp52bb_z6drbcbrhwl40000gn/T/oct-9fNAGX/baz.tgz: not 
in gzip format
dirlist(3): out of bound 2
error: called from
    install at line 102 column 19
    pkg at line 437 column 9
octave:1> pkg install file-name-with-typo.tgz
gzip: 
/var/folders/_4/9mx5ryp52bb_z6drbcbrhwl40000gn/T/oct-WzYajp/file-name-with-typo.tgz: 
not in gzip format
dirlist(3): out of bound 2
error: called from
    install at line 102 column 19
    pkg at line 437 column 9


I think this is because when "pkg install" is passed a nonexistent 
filename, it unconditionally tries to
download it as a URL. And because many ISPs are doing DNS hijacking 
these days, that download
will often succeed, but produce the HTML content of an ISP search page 
instead of a tarball.

I think this could be fixed by having pkg be a bit more conservative in 
how it interpreted its inputs,
and do some testing on the results of URL downloads. I've posted a patch 
here:
https://savannah.gnu.org/bugs/?55515. That will give you:

>> pkg install foobar
error: pkg: file not found: foobar.
This looks like an Octave Forge package name. Did you mean:
       pkg install -forge foobar
error: called from
    pkg at line 457 column 19
>> pkg install foobar.tgz
error: pkg: file not found: foobar.tgz
error: called from
    pkg at line 462 column 19
>> pkg install http://some-bogus-url/foo/bar/baz.tgz
error: pkg: Invalid package file downloaded from 
http://some-bogus-url/foo/bar/baz.tgz
File is HTML, not a tar archive.
error: called from
    pkg at line 445 column 23
>>

I think this is a nicer user experience, particularly for 
less-experienced users who might be
new to Octave and pkg.

Thoughts?

Cheers,
Andrew