Use setLevelDirError for level not found & parse errors

This exports the previously-internal setLevelDirError function in
FileSystemUtils and uses it for if a level is not found or there was a
parsing error. Previously, if a level failed to load in these ways, it
would take you to the error screen with no error, while printing it to
the console. But this makes it more user-friendly.

As a bonus, the text is localizable, just like the existing usage of
FILESYSTEM_setLevelDirError for if a path couldn't be mounted.

desktop_version/src/CustomLevels.cpp

@@ -1018,13 +1018,22 @@ bool customlevelclass::load(std::string _path)
 
     if (!FILESYSTEM_loadTiXml2Document(_path.c_str(), doc))
     {
-        vlog_warn("%s not found", _path.c_str());
+        FILESYSTEM_setLevelDirError(
+            loc::gettext("Level {path} not found"),
+            "path:str",
+            _path.c_str()
+        );
         goto fail;
     }
 
     if (doc.Error())
     {
-        vlog_error("Error parsing %s: %s", _path.c_str(), doc.ErrorStr());
+        FILESYSTEM_setLevelDirError(
+            loc::gettext("Error parsing {path}: {error}"),
+            "path:str, error:str",
+            _path.c_str(),
+            doc.ErrorStr()
+        );
         goto fail;
     }
 

desktop_version/src/FileSystemUtils.cpp

@@ -456,7 +456,7 @@ const char* FILESYSTEM_getLevelDirError(void)
     return levelDirError;
 }
 
-static void setLevelDirError(const char* text, const char* args_index, ...)
+void FILESYSTEM_setLevelDirError(const char* text, const char* args_index, ...)
 {
     levelDirHasError = true;
 
@@ -475,7 +475,7 @@ static bool FILESYSTEM_mountAssetsFrom(const char *fname)
 
     if (real_dir == NULL)
     {
-        setLevelDirError(
+        FILESYSTEM_setLevelDirError(
             loc::gettext("Could not mount {path}: real directory doesn't exist"),
             "path:str",
             fname

desktop_version/src/FileSystemUtils.h

@@ -60,6 +60,7 @@ void FILESYSTEM_freeEnumerate(EnumHandle* handle);
 bool FILESYSTEM_levelDirHasError(void);
 void FILESYSTEM_clearLevelDirError(void);
 const char* FILESYSTEM_getLevelDirError(void);
+void FILESYSTEM_setLevelDirError(const char* text, const char* args_index, ...);
 
 bool FILESYSTEM_openDirectoryEnabled(void);
 bool FILESYSTEM_openDirectory(const char *dname);