Catch potential errors on startup of Mantid

if .mantid or %appdata% directory permissions do not allow writing
re #14404

Framework/Kernel/src/ConfigService.cpp

@@ -255,10 +255,27 @@ ConfigServiceImpl::ConfigServiceImpl()
   Poco::Path path(appDataDir);
   path.pushDirectory("instrument");
   Poco::File file(path);
-  // createdirectories will fail gracefully if it is already present
-  file.createDirectories();
+  // createDirectories will fail gracefully if it is already present - but will
+  // throw an error if it cannot create the directory
+  try {
+    file.createDirectories();
+  } catch (Poco::FileException &fe) {
+    g_log.error()
+        << "Cannot create the local instrument cache directory ["
+        << path.toString()
+        << "]. Mantid will not be able to update instrument definitions.\n"
+        << fe.what() << std::endl;
+  }
   Poco::File vtpDir(getVTPFileDirectory());
-  vtpDir.createDirectories();
+  try {
+    vtpDir.createDirectories();
+  } catch (Poco::FileException &fe) {
+    g_log.error()
+        << "Cannot create the local instrument geometry cache directory ["
+        << path.toString()
+        << "]. Mantid will be slower at viewing complex instruments.\n"
+        << fe.what() << std::endl;
+  }
   // must update the cache of instrument paths
   cacheInstrumentPaths();
 }