Do not print a stacktrace when hitting a network timeout

[agateau-gg]

(This is a follow-up for #211)

GitGuardian Shield Version

• [x] I can reproduce this bug in the latest version

Command executed

See #211. ggshield scan ...

Describe the bug

As one can see in the description of #211, when ggshield hits a network timeout, it prints a Python stacktrace instead of showing a readable error message.

Expected behavior

In case of a network timeout, ggshield should print something along the lines of:

Error: network timeout while $action

Where $action can be "scanning file foo/bar" or "scanning commit 1234abcd"