ERC777: Add constraint to revert if hooks revert

This constraint was present implicitly and has been added explicitly for
clarity.

EIPS/eip-777.md

@@ -455,6 +455,10 @@ The token contract MUST `revert` when sending in any of the following cases:
 
 - Any of the resulting balances becomes negative, i.e. becomes less than zero (`0`).
 
+- The `tokensToSend` hook of the *holder* `revert`s.
+
+- The `tokensReceived` hook of the *recipient* `revert`s.
+
 The token contract MAY send tokens from many *holders*, to many *recipients*, or both. In this case:
 
 - The previous send rules MUST apply to all the *holders* and all the *recipients*.
@@ -598,6 +602,7 @@ The token contract MUST `revert` when minting in any of the following cases:
 - The resulting *recipient* balance after the mint is not a multiple of the *granularity* defined by the token contract.
 - The *recipient* is a contract, and it does not implement the `ERC777TokensRecipient` interface via [ERC1820].
 - The address of the *recipient* is `0x0`.
+- The `tokensReceived` hook of the *recipient* `revert`s.
 
 *NOTE*: The initial token supply at the creation of the token contract MUST be considered as minting
 for the amount of the initial supply to the address(es) receiving the initial supply.
@@ -687,6 +692,8 @@ The token contract MUST `revert` when burning in any of the following cases:
 
 - The address of the *holder* is `0x0`.
 
+- The `tokensToSend` hook of the *holder* `revert`s.
+
 *[ERC20] compatibility requirement*:  
 While a `Sent` event MUST NOT be emitted when burning;
 if the token contract is [ERC20] enabled, a `Transfer` event with the `to` parameter set to `0x0` SHOULD be emitted.