common/hexutil: wrap errors in json.UnmarshalTypeError

This adds type and struct field context to error messages.
Instead of "hex string of odd length" users will now see "json: cannot
unmarshal hex string of odd length into Go struct field SendTxArgs.from
of type common.Address".

common/types.go

@@ -31,6 +31,11 @@ const (
 	AddressLength = 20
 )
 
+var (
+	hashT    = reflect.TypeOf(Hash{})
+	addressT = reflect.TypeOf(Address{})
+)
+
 // Hash represents the 32 byte Keccak256 hash of arbitrary data.
 type Hash [HashLength]byte
 
@@ -72,6 +77,11 @@ func (h *Hash) UnmarshalText(input []byte) error {
 	return hexutil.UnmarshalFixedText("Hash", input, h[:])
 }
 
+// UnmarshalJSON parses a hash in hex syntax.
+func (h *Hash) UnmarshalJSON(input []byte) error {
+	return hexutil.UnmarshalFixedJSON(hashT, input, h[:])
+}
+
 // MarshalText returns the hex representation of h.
 func (h Hash) MarshalText() ([]byte, error) {
 	return hexutil.Bytes(h[:]).MarshalText()
@@ -194,6 +204,11 @@ func (a *Address) UnmarshalText(input []byte) error {
 	return hexutil.UnmarshalFixedText("Address", input, a[:])
 }
 
+// UnmarshalJSON parses a hash in hex syntax.
+func (a *Address) UnmarshalJSON(input []byte) error {
+	return hexutil.UnmarshalFixedJSON(addressT, input, a[:])
+}
+
 // UnprefixedHash allows marshaling an Address without 0x prefix.
 type UnprefixedAddress Address