Replace taproot_tweak_pubkey assertion with exception and add it to taproot_tweak_seckey too

bip-taproot.mediawiki

@@ -187,7 +187,8 @@ For any byte string <code>h</code> it holds that <code>taproot_tweak_pubkey(pubk
 <source lang="python">
 def taproot_tweak_pubkey(pubkey, h):
     t = int_from_bytes(tagged_hash("TapTweak", pubkey + h))
-    assert t < SECP256K1_ORDER
+    if t >= SECP256K1_ORDER:
+        raise ValueError
     Q = point_mul(point(pubkey), t)
     return bytes_from_int(x(Q)), is_quad(y(Q))
 
@@ -195,6 +196,8 @@ def taproot_tweak_seckey(seckey0, h):
     P = point_mul(G, int_from_bytes(seckey0))
     seckey = SECP256K1_ORDER - seckey0 if not is_quad(y(R)) else seckey
     t = int_from_bytes(tagged_hash("TapTweak", bytes_from_int(x(P)) + h))
+    if t >= SECP256K1_ORDER:
+        raise ValueError
     return (seckey + t) % SECP256K1_ORDER
 </source>