Please add a timeout for requests.get() calls

[xavpaice]

Checklist

• Confirmed this is an issue with charm-tools, not charmstore-client
• Provide versions of tools used
• Described the feature or ways to replicate the issue

What version am I running?

I ran the following command: snap info charm and got the following output:

name:      charm
summary:   charm and charm-tools
publisher: Canonical✓
store-url: https://snapcraft.io/charm
contact:   https://discourse.juju.is/c/charming
license:   unset
description: |
  charmstore-client and charm-tools
commands:
  - charm
snap-id:      2Rryoc2ylScfbFl4eQtpntHD9iuZuMvt
tracking:     latest/stable
refresh-date: 2021-02-02
channels:
  latest/stable:    2.8.2               2021-02-01 (609) 119MB classic
  latest/candidate: 2.8.2               2021-02-01 (609) 119MB classic
  latest/beta:      2.8.2               2021-02-01 (609) 119MB classic
  latest/edge:      2.8.3+git-1-736b1ad 2021-02-23 (620) 119MB classic
installed:          2.8.2                          (609) 119MB classic

I am using: Ubuntu 21.04

Issue/Feature

I expect/expected the following

When network issues prevent access to the charm layer index, I expect to see a timeout and an error message explaining what was wrong.

What I got

If I do not set appropriate environment variables (for proxy):

build: Unable to locate layer:apt. Do you need to set CHARM_LAYERS_DIR?

If I set the HTTP_PROXY, HTTPS_PROXY, and NO_PROXY variables, instead of that message the build (with debug enabled) hangs at:

charmtools.build.fetchers: Checking layer index: https://juju.github.io/layer-index/layers/apt.json

Regardless of the difficulties with network access, it would be good if the build step could add a timeout and more clear exception handling at

charm-tools/charmtools/build/fetchers.py

Line 129 in 736b1ad

result = requests.get(uri)

as that would aid charm developers trying to fix their build environment.