Don't fail completely if we throw on a table.

[jcjones]

This is super hard to test. I'd need to probably move to using SQLite.

[jcjones]

Sample stack trace:

6]: Traceback (most recent call last):
6]:   File "/usr/local/bin/partition-manager", line 11, in <module>
6]:     load_entry_point('mariadb-sequential-partition-manager==0.3.2', 'console_scripts', 'partition-manager')()
6]:   File "/usr/lib/python3/dist-packages/partitionmanager/cli.py", line 475, in main
6]:     raise e
6]:   File "/usr/lib/python3/dist-packages/partitionmanager/cli.py", line 461, in main
6]:     output = args.func(args)
6]:   File "/usr/lib/python3/dist-packages/partitionmanager/cli.py", line 194, in partition_cmd
6]:     return do_partition(conf)
6]:   File "/usr/lib/python3/dist-packages/partitionmanager/cli.py", line 323, in do_partition
6]:     sql_cmds = pm_tap.get_pending_sql_reorganize_partition_commands(
6]:   File "/usr/lib/python3/dist-packages/partitionmanager/table_append_partition.py", line 779, in get_pending_sql_reorganize_partition_commands
6]:     partition_changes = _plan_partition_changes(
6]:   File "/usr/lib/python3/dist-packages/partitionmanager/table_append_partition.py", line 541, in _plan_partition_changes
6]:     rates = _get_weighted_position_increase_per_day_for_partitions(
6]:   File "/usr/lib/python3/dist-packages/partitionmanager/table_append_partition.py", line 288, in _get_weighted_position_increase_per_day_for_partitions
6]:     raise ValueError("No valid rates of change")
6]: ValueError: No valid rates of change