Improve error message for when status db is locked

Hello, lately I can't successfully vdirsyncer sync my calendars because of a
database is locked error. Synchronization works fine with android's davdroid which talks with the same server.

Your vdirsyncer version: 0.16.4
If applicable, which server software (and which version) you're using: Nextcloud 13
Your Python version: 3.6.4
Your operating system: Arch linux
Relevant debug output:

Syncing my_calendars/jucy
debug:   File "/usr/lib/python3.6/site-packages/vdirsyncer/cli/tasks.py", line 63, in sync_collection
debug:     collection.name) as status:
debug:   File "/usr/lib/python3.6/contextlib.py", line 81, in __enter__
debug:     return next(self.gen)
debug:   File "/usr/lib/python3.6/site-packages/vdirsyncer/cli/utils.py", line 227, in manage_sync_status
debug:     status = SqliteStatus(path)
debug:   File "/usr/lib/python3.6/site-packages/vdirsyncer/sync/status.py", line 119, in __init__
debug:     self._update_schema()
debug:   File "/usr/lib/python3.6/site-packages/vdirsyncer/sync/status.py", line 127, in _update_schema
debug:     with _exclusive_transaction(self._c) as c:
debug:   File "/usr/lib/python3.6/contextlib.py", line 81, in __enter__
debug:     return next(self.gen)
debug:   File "/usr/lib/python3.6/site-packages/vdirsyncer/sync/status.py", line 13, in _exclusive_transaction
debug:     c = conn.execute('BEGIN EXCLUSIVE TRANSACTION')
Syncing my_calendars/personal
debug:   File "/usr/lib/python3.6/site-packages/vdirsyncer/cli/tasks.py", line 63, in sync_collection
debug:     collection.name) as status:
debug:   File "/usr/lib/python3.6/contextlib.py", line 81, in __enter__
debug:     return next(self.gen)
debug:   File "/usr/lib/python3.6/site-packages/vdirsyncer/cli/utils.py", line 227, in manage_sync_status
debug:     status = SqliteStatus(path)
debug:   File "/usr/lib/python3.6/site-packages/vdirsyncer/sync/status.py", line 119, in __init__
debug:     self._update_schema()
debug:   File "/usr/lib/python3.6/site-packages/vdirsyncer/sync/status.py", line 127, in _update_schema
debug:     with _exclusive_transaction(self._c) as c:
debug:   File "/usr/lib/python3.6/contextlib.py", line 81, in __enter__
debug:     return next(self.gen)
debug:   File "/usr/lib/python3.6/site-packages/vdirsyncer/sync/status.py", line 13, in _exclusive_transaction
debug:     c = conn.execute('BEGIN EXCLUSIVE TRANSACTION')

Start with the failing path in vdirsyncer/cli/tasks.py and vdirsyncer/cli/utils.py, then inspect SqliteStatus and _exclusive_transaction in vdirsyncer/sync/status.py. Reproduce the locked status-database failure and determine the expected improved error message; done means the failure reports the problem clearly without obscuring the original context.