Updatd will_run_on_cluster to now warn if to_cluster is active but DR…

…MAA is not present

cgatcore/pipeline/execution.py

@@ -440,11 +440,14 @@ def join_statements(statements, infile, outfile=None):
 
 
 def will_run_on_cluster(options):
-    run_on_cluster = options.get("to_cluster", True) and \
-        not options.get("without_cluster", False) and \
-        HAS_DRMAA and \
-        GLOBAL_SESSION is not None
-    return run_on_cluster
+    wants_cluster = options.get("to_cluster", True) and \
+        not options.get("without_cluster", False)
+
+    if wants_cluster and not HAS_DRMAA:
+        raise ValueError("Cluster execution requested (to_cluster=True) but DRMAA library is not available. "
+                       "Please install drmaa package in your environment to enable cluster execution.")
+
+    return wants_cluster and HAS_DRMAA and GLOBAL_SESSION is not None
 
 
 class Executor(object):