rpc: check and throw specific pubkey parsing errors in HexToPubKey

In the helper `HexToPubKey`, check for three different causes of legacy
public key parsing errors (in this order):

    - pubkey is not a hex string
    - pubkey doesn't have a valid length (33 or 65 bytes) [NEW]
    - pubkey is cryptographically invalid, i.e. not on curve
      (`IsFullyValid` check)

and throw a specific error message for each one. Note that the error
code is identical for all of them (-5), so this doesn't break RPC API
compatibility.

The helper is currently used for the RPCs `createmultisig` and
`addmultisigaddress`. The length checks can be removed from the
call-sites and error message checks in the functional tests are adapted.

test/functional/rpc_rawtransaction.py

@@ -491,11 +491,11 @@ class RawTransactionsTest(BitcoinTestFramework):
         addr2Obj = self.nodes[2].getaddressinfo(addr2)
 
         # Tests for createmultisig and addmultisigaddress
-        assert_raises_rpc_error(-5, "Invalid public key", self.nodes[0].createmultisig, 1, ["01020304"])
+        assert_raises_rpc_error(-5, 'Pubkey "01020304" must have a length of either 33 or 65 bytes', self.nodes[0].createmultisig, 1, ["01020304"])
         # createmultisig can only take public keys
         self.nodes[0].createmultisig(2, [addr1Obj['pubkey'], addr2Obj['pubkey']])
         # addmultisigaddress can take both pubkeys and addresses so long as they are in the wallet, which is tested here
-        assert_raises_rpc_error(-5, "Invalid public key", self.nodes[0].createmultisig, 2, [addr1Obj['pubkey'], addr1])
+        assert_raises_rpc_error(-5, f'Pubkey "{addr1}" must be a hex string', self.nodes[0].createmultisig, 2, [addr1Obj['pubkey'], addr1])
 
         mSigObj = self.nodes[2].addmultisigaddress(2, [addr1Obj['pubkey'], addr1])['address']