Remove sys.exit()

Library code should not call exit.

ligo/computeDiskMass.py

@@ -16,11 +16,8 @@ def compute_isco(chi_bh):
     returns the innermost stable circular orbit radius normalized by the mass
     of the BH.
     '''
-    try:
-        assert np.all([np.abs(chi_bh) <= 1.0])
-    except AssertionError:
-        print('|chi1| must be less than 1.0')
-        sys.exit(1)
+    if not np.all([np.abs(chi_bh) <= 1.0]):
+        raise ValueError('|chi1| must be less than 1.0')
     Z1 = 1.0 + ((1.0 - chi_bh**2)**(1./3.))*((1 + chi_bh)**(1./3.) +
                                              (1 - chi_bh)**(1./3.))
     Z2 = np.sqrt(3*chi_bh**2 + Z1**2)