add check for fsldir and update aws join

openneuro-dl/openneuro_dl

@@ -77,7 +77,12 @@ def main():
         print("ERROR: an openneuro dataset id must start with 'ds'. Please check your dataset id")
         sys.exit(1)
 
-    awsbin=join( getenv('FSLDIR'),"fslpython/envs/fslpython/bin","aws")
+    FSLDIR = getenv('FSLDIR') # returns none if not found
+    if FSLDIR is None:
+        print("ERROR: Unable to find FSLDIR evironment variable")
+        sys.exit(1)
+
+    awsbin = join(FSLDIR, "fslpython", "envs", "fslpython", "bin", "aws")
 
     # use subprocess.run to call the aws command to sync from the aws bucket (download the dataset)
     run(