Add check on control file and graceful exit, (#196)

Modity `mon_data_space.py` adding a check on the control file and
graceful exit if not found. Without this exit the entire plot job
terminates. With this exit only the affected thread halts (with error
message), allowing all other plots to proceed.

This has been tested on wcoss2 by removing some control files from the
test data set. Missing control files are reported as warnings and all
plots with valid control files run to completion.

## Dependencies

None

Resolves #191 .

src/eva/data/mon_data_space.py

@@ -63,6 +63,10 @@ def execute(self, dataset_config, data_collections, timing):
         # Get control file and parse
         # --------------------------
         control_file = get(dataset_config, self.logger, 'control_file')
+        if not os.path.isfile(control_file[0]):
+            self.logger.info(f'Warning: control file {control_file[0]} not found, unable' +
+                             ' to plot')
+            exit(1)
 
         dims_arr = []
         if self.is_stn_data(control_file[0]):