ignore exceptions that happen during anonymous tracking

[drewbanin]

Issue

Issue description

Anonymous event tracking should never impact deployments of dbt. If exceptions occur during the tracking of anonymous events, dbt should log them to the debug logs and carry on with tracking disabled for the rest of the run.

Example traceback:

[Errno 13] Permission denied: '/.dbt'
Traceback (most recent call last):
  File "/usr/local/lib/python3.7/site-packages/dbt/main.py", line 76, in main
    results, succeeded = handle_and_check(args)
  File "/usr/local/lib/python3.7/site-packages/dbt/main.py", line 120, in handle_and_check
    dbt.tracking.initialize_tracking()
  File "/usr/local/lib/python3.7/site-packages/dbt/tracking.py", line 275, in initialize_tracking
    active_user.initialize()
  File "/usr/local/lib/python3.7/site-packages/dbt/tracking.py", line 51, in initialize
    cookie = self.get_cookie()
  File "/usr/local/lib/python3.7/site-packages/dbt/tracking.py", line 71, in get_cookie
    user = self.set_cookie()
  File "/usr/local/lib/python3.7/site-packages/dbt/tracking.py", line 62, in set_cookie
    dbt.clients.system.make_directory(cookie_dir)
  File "/usr/local/lib/python3.7/site-packages/dbt/clients/system.py", line 85, in make_directory
    raise e
  File "/usr/local/lib/python3.7/site-packages/dbt/clients/system.py", line 79, in make_directory
    os.makedirs(path)
  File "/usr/local/lib/python3.7/os.py", line 221, in makedirs
    mkdir(name, mode)
PermissionError: [Errno 13] Permission denied: '/.dbt'

System information

The output of dbt --version:

0.12.1

The operating system you're running on:
Linux

The python version you're using (probably the output of python --version)
3.7