rpc: Improve error message when snapshot file does not exist

The current message may be a bit misleading, since it just mentions
permissions, but not the file actually existing.

Add functional tests for the new error and for exercising the
permission error.

src/rpc/blockchain.cpp

@@ -3259,6 +3259,12 @@ static RPCHelpMan loadtxoutset()
     ChainstateManager& chainman = EnsureChainman(node);
     const fs::path path{AbsPathForConfigVal(EnsureArgsman(node), fs::u8path(self.Arg<std::string>("path")))};
 
+    if (!fs::exists(path)) {
+        throw JSONRPCError(
+            RPC_INVALID_PARAMETER,
+            "Txoutset snapshot file " + path.utf8string() + " does not exist.");
+    }
+
     FILE* file{fsbridge::fopen(path, "rb")};
     AutoFile afile{file};
     if (afile.IsNull()) {

test/functional/feature_assumeutxo.py

@@ -10,6 +10,7 @@
 `CRegTestParams::m_assumeutxo_data` in `src/kernel/chainparams.cpp`.
 """
 from shutil import rmtree
+import platform
 
 from dataclasses import dataclass
 from test_framework.blocktools import (
@@ -190,7 +191,16 @@ def test_invalid_file_path(self):
         self.log.info("Test bitcoind should fail when file path is invalid.")
         node = self.nodes[0]
         path = node.datadir_path / node.chain / "invalid" / "path"
+        assert_raises_rpc_error(-8, "Txoutset snapshot file {} does not exist.".format(path), node.loadtxoutset, path)
+
+    def test_invalid_file_permissions(self):
+        self.log.info("Test bitcoind should fail when file has no read permissions.")
+        node = self.nodes[0]
+        path = node.datadir_path / "bad_permission"
+        path.mkdir()
+        path.chmod(0)
         assert_raises_rpc_error(-8, "Couldn't open file {} for reading.".format(path), node.loadtxoutset, path)
+        path.rmdir()
 
     def test_snapshot_with_less_work(self, dump_output_path):
         self.log.info("Test bitcoind should fail when snapshot has less accumulated work than this node.")
@@ -455,6 +465,8 @@ def check_dump_output(output):
         self.test_invalid_snapshot_scenarios(dump_output['path'])
         self.test_invalid_chainstate_scenarios()
         self.test_invalid_file_path()
+        if platform.system() != "Windows":
+            self.test_invalid_file_permissions()
         self.test_snapshot_block_invalidated(dump_output['path'])
         self.test_snapshot_not_on_most_work_chain(dump_output['path'])