[BUG] Confusing traceback on invalid file

[oerc0122]

Description of the error
On providing an invalid file to any IJob, the traceback refers to an KeyError in selection_length rather than stating the file doesn't exist. While this is reported in the logs, so MDANSE apparantly knows.

Describe the expected result
A FileNotFoundError raised with a useful message.

Describe the actual result

Log

Could not use xxxx as input trajectory
IJob did not find 'write_logs' in output_files
Job failed with traceback: Traceback (most recent call last):
  File "/home/jacob/MDANSE/MDANSE/Src/MDANSE/Framework/Jobs/IJob.py", line 395, in run
    self.initialize()
  File "/home/jacob/MDANSE/MDANSE/Src/MDANSE/Framework/Jobs/VelocityAutoCorrelationFunction.py", line 114, in initialize
    self.numberOfSteps = self.configuration["atom_selection"]["selection_length"]
KeyError: 'selection_length'

Traceback (most recent call last):
  File "/home/jacob/MDANSE/MDANSE/Src/MDANSE/Framework/Jobs/IJob.py", line 395, in run
    self.initialize()
  File "/home/jacob/MDANSE/MDANSE/Src/MDANSE/Framework/Jobs/VelocityAutoCorrelationFunction.py", line 114, in initialize
    self.numberOfSteps = self.configuration["atom_selection"]["selection_length"]
KeyError: 'selection_length'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/home/jacob/MDANSE/MDANSE/Tests/UnitTests/Analysis/tmp.py", line 15, in <module>
    vacf.run(parameters, status=True)
  File "/home/jacob/MDANSE/MDANSE/Src/MDANSE/Framework/Jobs/IJob.py", line 418, in run
    raise JobError(self, tb)
MDANSE.Framework.Jobs.IJob.JobError: Traceback (most recent call last):
  File "/home/jacob/MDANSE/MDANSE/Src/MDANSE/Framework/Jobs/IJob.py", line 395, in run
    self.initialize()
  File "/home/jacob/MDANSE/MDANSE/Src/MDANSE/Framework/Jobs/VelocityAutoCorrelationFunction.py", line 114, in initialize
    self.numberOfSteps = self.configuration["atom_selection"]["selection_length"]
KeyError: 'selection_length'

Suggested fix
If trying to load a necessary file, raise early.

Additional details

MWE

from MDANSE.Framework.Jobs.IJob import IJob

temp_name = "output"

parameters = {
    "frames": (0, 10, 1, 5),
    "interpolation_order": 1,
    "output_files": (temp_name, ("MDAFormat",), "INFO"),
    "running_mode": ("single-core",),
    "normalize": True,
    "trajectory": "xxxx",
}

vacf = IJob.create("VelocityAutoCorrelationFunction")
vacf.run(parameters, status=True)