Nicer YAML configuration error messages

[maxfischer2781]

Malformed YAML configuration files (including unsafe features) currently kill the daemon with a raw traceback (see #53). A human-readable error and possibly help message should be provided instead.

Better error message:

2019-10-02 14:55:40 (601034) Using configuration cobald_test.yml
2019-10-02 14:55:40 (601034) Failed to load cobald_test.yml:
SafeYAML does not support tag '!!python/object/apply:tardis.resources.poolfactory.create_composite_pool'
  in "cobald_test.yml", line 13, column 5

Better help message:

2019-10-02 14:55:40 (601034) Using configuration cobald_test.yml
2019-10-02 14:55:40 (601034) Failed to load cobald_test.yml:

COBalD YAML configuration files do not support arbitrary '!!python/object/apply' targets

11|    high_allocation: 0.90
12|    rate: 1
13|  - !!python/object/apply:tardis.resources.poolfactory.create_composite_pool
       ^^^^^^^^^^^^^^^^^^^^^

The tag target 'tardis.resources.poolfactory.create_composite_pool' is not safe for loading.
Register it as a plugin or mark it as safe.

HINT: See the configuration documentation at https://cobald.readthedocs.io/en/latest/source/daemon/config.html