builder: fix missing default value for details in ComposeStatusError

The builder plugin could produce a traceback when image build failed,
but it didn't contain any details.

```
Traceback (most recent call last):
  File "/usr/lib/python3.6/site-packages/koji/daemon.py", line 1468, in runTask
    response = (handler.run(),)
  File "/usr/lib/python3.6/site-packages/koji/tasks.py", line 335, in run
    return koji.util.call_with_argcheck(self.handler, self.params, self.opts)
  File "/usr/lib/python3.6/site-packages/koji/util.py", line 271, in call_with_argcheck
    return func(*args, **kwargs)
  File "/usr/lib/koji-builder-plugins/osbuild.py", line 719, in handler
    status = client.wait_for_compose(cid, callback=self.on_status_update)
  File "/usr/lib/koji-builder-plugins/osbuild.py", line 483, in wait_for_compose
    status = self.compose_status(compose_id)
  File "/usr/lib/koji-builder-plugins/osbuild.py", line 454, in compose_status
    return ComposeStatus.from_dict(res.json())
  File "/usr/lib/koji-builder-plugins/osbuild.py", line 251, in from_dict
    ImageStatus.from_dict(s) for s in data["image_statuses"]
  File "/usr/lib/koji-builder-plugins/osbuild.py", line 251, in <listcomp>
    ImageStatus.from_dict(s) for s in data["image_statuses"]
  File "/usr/lib/koji-builder-plugins/osbuild.py", line 228, in from_dict
    error = ComposeStatusError(error_id=error_id, **error)
TypeError: __init__() missing 1 required positional argument: 'details'
```

Signed-off-by: Tomáš Hozza <thozza@redhat.com>

plugins/builder/osbuild.py

@@ -187,7 +187,7 @@ def as_dict(self):
 
 
 class ComposeStatusError:
-    def __init__(self, error_id: int, reason: str, details: Optional[Dict]):
+    def __init__(self, error_id: int, reason: str, details: Optional[Dict]=None):
         self.error_id = error_id
         self.reason = reason
         self.details = details