Fixed #102: Report useful error message when fetch fails due to missing VCS tool

[johnsca]

build: Composing into /home/ubuntu/t
build: Unable to run "git": No such file or directory

[marcoceppi]

msg = 'Unable to run "%s": %s' % (args[0], e.strerror)
if e.errno == 127:
    msg = msg + '\nPlease install "%s" before continuing.' % args[0]
raise FetchError(msg)

That way the user has an actionable item?

[marcoceppi]

⛵️ LGTM