qt: Update some messages after suggestions by translators

- *cannot* is more common, thus preferred to *can not*
- Use *Watch-only* instead of *Watchonly* as one word

src/qt/intro.cpp

@@ -182,7 +182,7 @@ void Intro::pickDataDirectory()
                 break;
             } catch(fs::filesystem_error &e) {
                 QMessageBox::critical(0, tr("Bitcoin"),
-                    tr("Error: Specified data directory \"%1\" can not be created.").arg(dataDir));
+                    tr("Error: Specified data directory \"%1\" cannot be created.").arg(dataDir));
                 /* fall through, back to choosing screen */
             }
         }