Better error from workflows when LYMAN_DIR is not defined

[mwaskom]

Presently one gets this unhelpful message

(lyman-dev) [kianilab-cs1 data]$ lyman template -s s1
171025-10:41:43,806 workflow INFO:
	 Workflow template settings: ['check', 'execution', 'logging']
171025-10:41:43,815 workflow INFO:
	 Running in parallel.
Traceback (most recent call last):
  File "/home/mwaskom/anaconda/envs/lyman-dev/lib/python3.6/site-packages/nipype/pipeline/plugins/base.py", line 376, in _generate_dependency_list
    format='lil')
  File "/home/mwaskom/anaconda/envs/lyman-dev/lib/python3.6/site-packages/networkx/convert_matrix.py", line 784, in to_scipy_sparse_matrix
    raise nx.NetworkXError("Graph has no nodes or edges")
networkx.exception.NetworkXError: Graph has no nodes or edges

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/home/mwaskom/anaconda/envs/lyman-dev/bin/lyman", line 6, in <module>
    exec(compile(open(__file__).read(), __file__, 'exec'))
  File "/home/mwaskom/code/lyman/scripts/lyman", line 123, in <module>
    lyman.execute(wf, args, info)
  File "/home/mwaskom/code/lyman/lyman/frontend.py", line 433, in execute
    res = wf.run(plugin, plugin_args)
  File "/home/mwaskom/anaconda/envs/lyman-dev/lib/python3.6/site-packages/nipype/pipeline/engine/workflows.py", line 590, in run
    runner.run(execgraph, updatehash=updatehash, config=self.config)
  File "/home/mwaskom/anaconda/envs/lyman-dev/lib/python3.6/site-packages/nipype/pipeline/plugins/base.py", line 122, in run
    self._generate_dependency_list(graph)
  File "/home/mwaskom/anaconda/envs/lyman-dev/lib/python3.6/site-packages/nipype/pipeline/plugins/base.py", line 379, in _generate_dependency_list
    nodelist=self.procs)
  File "/home/mwaskom/anaconda/envs/lyman-dev/lib/python3.6/site-packages/networkx/convert_matrix.py", line 784, in to_scipy_sparse_matrix
    raise nx.NetworkXError("Graph has no nodes or edges")
networkx.exception.NetworkXError: Graph has no nodes or edges