wallet: remove fetch pre-selected-inputs responsibility from SelectCoins

so if there is an error in any of the pre-set coins, we can fail right away
without computing the wallet available coins set (calling `AvailableCoins`)
which is a slow operation as it goes through the entire wallet's txes map.

----------------------

And to make the Coin Selection flow cleared, have decoupled SelectCoins in two functions:

1) AutomaticCoinSelection.
2) SelectCoins.

1) AutomaticCoinSelection:
   Receives a set of coins and selects the best subset of them to
   cover the target amount.

2) SelectCoins
   In charge of select all the user manually selected coins first ("pre-set inputs"), and
   if coin_control 'm_allow_other_inputs=true', call 'AutomaticCoinSelection' to select a
   subset of coins owned by the wallet to cover for the target - preset_inputs.total_amount
   remaining value.

test/functional/rpc_fundrawtransaction.py

@@ -406,7 +406,9 @@ class RawTransactionsTest(BitcoinTestFramework):
 
     def test_invalid_input(self):
         self.log.info("Test fundrawtxn with an invalid vin")
-        inputs  = [ {'txid' : "1c7f966dab21119bac53213a2bc7532bff1fa844c124fd750a7d0b1332440bd1", 'vout' : 0} ] #invalid vin!
+        txid = "1c7f966dab21119bac53213a2bc7532bff1fa844c124fd750a7d0b1332440bd1"
+        vout = 0
+        inputs  = [ {'txid' : txid, 'vout' : vout} ] #invalid vin!
         outputs = { self.nodes[0].getnewaddress() : 1.0}
         rawtx   = self.nodes[2].createrawtransaction(inputs, outputs)
         assert_raises_rpc_error(-4, "Unable to find UTXO for external input", self.nodes[2].fundrawtransaction, rawtx)
@@ -1011,7 +1013,7 @@ class RawTransactionsTest(BitcoinTestFramework):
 
         # An external input without solving data should result in an error
         raw_tx = wallet.createrawtransaction([ext_utxo], {self.nodes[0].getnewaddress(): ext_utxo["amount"] / 2})
-        assert_raises_rpc_error(-4, "Insufficient funds", wallet.fundrawtransaction, raw_tx)
+        assert_raises_rpc_error(-4, "Not solvable pre-selected input COutPoint(%s, %s)" % (ext_utxo["txid"][0:10], ext_utxo["vout"]), wallet.fundrawtransaction, raw_tx)
 
         # Error conditions
         assert_raises_rpc_error(-5, "'not a pubkey' is not hex", wallet.fundrawtransaction, raw_tx, {"solving_data": {"pubkeys":["not a pubkey"]}})