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/util/system.cpp

@@ -438,27 +438,6 @@ const fs::path& ArgsManager::GetDataDir(bool net_specific) const
     return path;
 }
 
-void ArgsManager::EnsureDataDir() const
-{
-    /**
-     * "/wallets" subdirectories are created in all **new**
-     * datadirs, because wallet code will create new wallets in the "wallets"
-     * subdirectory only if exists already, otherwise it will create them in
-     * the top-level datadir where they could interfere with other files.
-     * Wallet init code currently avoids creating "wallets" directories itself
-     * for backwards compatibility, but this be changed in the future and
-     * wallet code here could go away (#16220).
-     */
-    auto path{GetDataDir(false)};
-    if (!fs::exists(path)) {
-        fs::create_directories(path / "wallets");
-    }
-    path = GetDataDir(true);
-    if (!fs::exists(path)) {
-        fs::create_directories(path / "wallets");
-    }
-}
-
 void ArgsManager::ClearPathCache()
 {
     LOCK(cs_args);
@@ -502,25 +481,6 @@ bool ArgsManager::IsArgSet(const std::string& strArg) const
     return !GetSetting(strArg).isNull();
 }
 
-bool ArgsManager::InitSettings(std::string& error)
-{
-    EnsureDataDir();
-    if (!GetSettingsPath()) {
-        return true; // Do nothing if settings file disabled.
-    }
-
-    std::vector<std::string> errors;
-    if (!ReadSettingsFile(&errors)) {
-        error = strprintf("Failed loading settings file:\n%s\n", MakeUnorderedList(errors));
-        return false;
-    }
-    if (!WriteSettingsFile(&errors)) {
-        error = strprintf("Failed saving settings file:\n%s\n", MakeUnorderedList(errors));
-        return false;
-    }
-    return true;
-}
-
 bool ArgsManager::GetSettingsPath(fs::path* filepath, bool temp, bool backup) const
 {
     fs::path settings = GetPathArg("-settings", BITCOIN_SETTINGS_FILENAME);

src/util/system.h

@@ -434,13 +434,6 @@ protected:
      */
     std::optional<unsigned int> GetArgFlags(const std::string& name) const;
 
-    /**
-     * Read and update settings file with saved settings. This needs to be
-     * called after SelectParams() because the settings file location is
-     * network-specific.
-     */
-    bool InitSettings(std::string& error);
-
     /**
      * Get settings file path, or return false if read-write settings were
      * disabled with -nosettings.
@@ -480,12 +473,6 @@ protected:
      */
     void LogArgs() const;
 
-    /**
-     * If datadir does not exist, create it along with wallets/
-     * subdirectory(s).
-     */
-    void EnsureDataDir() const;
-
 private:
     /**
      * Get data directory path