Deduplicate bitcoind and bitcoin-qt init code

Add common InitConfig function to deduplicate bitcoind and bitcoin-qt code
reading config files and creating the datadir.

There are a few minor changes in behavior:

- In bitcoin-qt, when there is a problem reading the configuration file, the
  GUI error text has changed from "Error: Cannot parse configuration file:" to
  "Error reading configuration file:" to be consistent with bitcoind.
- In bitcoind, when there is a problem reading the settings.json file, the
  error text has changed from "Failed loading settings file" to "Settings
  file could not be read" to be consistent with bitcoin-qt.
- In bitcoind, when there is a problem writing the settings.json file, the
  error text has changed from "Failed saving settings file" to "Settings file
  could not be written" to be consistent with bitcoin-qt.
- In bitcoin-qt, if there datadir is not accessible (e.g. no permission to read),
  there is an normal error dialog showing "Error: filesystem error: status:
  Permission denied [.../settings.json]", instead of an uncaught exception

src/bitcoind.cpp

@@ -9,6 +9,7 @@
 
 #include <chainparams.h>
 #include <clientversion.h>
+#include <common/init.h>
 #include <common/url.h>
 #include <compat/compat.h>
 #include <init.h>
@@ -150,17 +151,8 @@ static bool AppInit(NodeContext& node, int argc, char* argv[])
     std::any context{&node};
     try
     {
-        if (!CheckDataDirOption(args)) {
-            return InitError(Untranslated(strprintf("Specified data directory \"%s\" does not exist.", args.GetArg("-datadir", ""))));
-        }
-        if (!args.ReadConfigFiles(error, true)) {
-            return InitError(Untranslated(strprintf("Error reading configuration file: %s", error)));
-        }
-        // Check for chain settings (Params() calls are only valid after this clause)
-        try {
-            SelectParams(args.GetChainName());
-        } catch (const std::exception& e) {
-            return InitError(Untranslated(strprintf("%s", e.what())));
+        if (auto error = common::InitConfig(args)) {
+            return InitError(error->message, error->details);
         }
 
         // Error out when loose non-argument tokens are encountered on command line
@@ -170,11 +162,6 @@ static bool AppInit(NodeContext& node, int argc, char* argv[])
             }
         }
 
-        if (!args.InitSettings(error)) {
-            InitError(Untranslated(error));
-            return false;
-        }
-
         // -server defaults to true for bitcoind but not for the GUI so do this here
         args.SoftSetBoolArg("-server", true);
         // Set this early so that parameter interactions go to console