Add option to purge workspace that could not be loaded

[rezib]

For example:

remi@build:~$ fatbuildrctl history purge
Submitted tasks history purge task 27a21814-21ec-4d05-9828-4f5f64f6bc73
 ⚬ INFO: Running history purge task 27a21814-21ec-4d05-9828-4f5f64f6bc73
 ⚬ ERROR: Unable to load unsupported task /var/lib/fatbuildr/workspaces/devs/fcb189cd-5033-4225-b5b1-109ffc0b8783: 'TaskForm' object has no attribute 'result'
 ⚬ ERROR: Unable to load unsupported task /var/lib/fatbuildr/workspaces/devs/f4eddb0f-2892-466a-a083-cb32c2dbd43c: 'TaskForm' object has no attribute 'result'
 ⚬ ERROR: Unable to load unsupported task /var/lib/fatbuildr/workspaces/devs/d1ca83f5-1fb7-4e58-9d7e-99823acd6efa: 'TaskForm' object has no attribute 'result'
 ⚬ ERROR: Unable to load unsupported task /var/lib/fatbuildr/workspaces/devs/845d5a46-677a-495a-a410-bd04fbdbf0cd: 'TaskForm' object has no attribute 'result'
 ⚬ ERROR: Unable to load unsupported task /var/lib/fatbuildr/workspaces/devs/7f3f0cc8-1d19-4283-ad4d-628a857edbc2: 'TaskForm' object has no attribute 'result'
 ⚬ ERROR: Unable to load unsupported task /var/lib/fatbuildr/workspaces/devs/d9e5662f-dafc-404a-b989-15c19300897b: 'TaskForm' object has no attribute 'result'
 ⚬ ERROR: Unable to load unsupported task /var/lib/fatbuildr/workspaces/devs/14738a99-d65b-46eb-87ab-ee4f1f66c4f1: 'TaskForm' object has no attribute 'result'
 ⚬ ERROR: Unable to load unsupported task /var/lib/fatbuildr/workspaces/devs/e53b1180-0241-4438-ac7c-d1e5403c9975: 'TaskForm' object has no attribute 'result'
 ⚬ ERROR: Unable to load unsupported task /var/lib/fatbuildr/workspaces/devs/59631ea9-6ac4-4d01-9265-cd8036421e37: 'TaskForm' object has no attribute 'result'
 ⚬ ERROR: Unable to load unsupported task /var/lib/fatbuildr/workspaces/devs/bf69eaa3-d4d5-4d31-8f43-620843966def: 'TaskForm' object has no attribute 'result'
 ⚬ ERROR: Unable to load unsupported task /var/lib/fatbuildr/workspaces/devs/4e60a46e-e812-4f42-a219-659cda57d4f8: 'TaskForm' object has no attribute 'result'
…

It should be possible to purge these workspaces as well.