Raise ValueError on slurm_update_reservation (#257)

To be consistent with other functions and methods, we need to raise a
ValueError instead of just returning the error code. This also allows
client applications to more easily interpret the reason for the error.

Author: pllopis

pyslurm/pyslurm.pyx

@@ -4370,6 +4370,9 @@ def slurm_update_reservation(dict reservation_dict={}):
         resv_msg.flags = int_value
 
     errCode = slurm.slurm_update_reservation(&resv_msg)
+    if errCode != 0:
+        apiError = slurm.slurm_get_errno()
+        raise ValueError(slurm.stringOrNone(slurm.slurm_strerror(apiError), ''), apiError)
 
     return errCode