Chunk error string to make it better translatable.

src/main.cpp

@@ -112,7 +112,8 @@ int main(int argc, char *argv[])
                 }
 
                 cerr << '\n'
-                     << format(translate("ERROR: Could not open {0:s}: {1:s}")
+                     << translate("ERROR: ")
+                     << format(translate("Could not open {0:s}: {1:s}")
                                    .str()
                                    .data(),
                                e.path1(), e.what())