Passing background/resolution with empty source assigned gives unhelpful error

[alexhroom]

If a Project is created and then a background or resolution source is set to "", then the error is

Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
 File "/home/rec93639/Code/python-RAT/RATapi/run.py", line 112, in run
 problem_definition, limits, cpp_controls = make_input(project, controls)
 File "/home/rec93639/Code/python-RAT/RATapi/inputs.py", line 148, in make_input
 problem = make_problem(project)
 File "/home/rec93639/Code/python-RAT/RATapi/inputs.py", line 283, in make_problem
 contrast_resolution_param.append(project.resolution_parameters.index(resolution.source, True))
 File "/home/rec93639/Code/python-RAT/RATapi/classlist.py", line 287, in index
 return self.data.index(item, *args) + int(offset)
ValueError: '' is not in list

rather than telling the user they need to define source.

MRE:

import RATapi as RAT
p, r = RAT.examples.DSPC_standard_layers()

p.resolutions[0].source = ""

c = RAT.Controls()
RAT.run(p, c)