Catch error instead of 500

[paulineribeyre]

Catch this error that causes Sheepdog to return a 500, and return a 400 instead:

  File "/sheepdog/sheepdog/blueprint/routes/views/program/__init__.py", line 198, in create_project
    node.props.update(doc)
  File "/usr/local/lib/python3.6/site-packages/psqlgraph/attributes.py", line 76, in update
    raise AttributeError("{} has no property {}".format(self.source, key))
AttributeError: <Project(029d0e6b-781b-52cc-ba36-748639545662)> has no property programs

[PlanXCyborg]

The style in this PR agrees with black. ✔️

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