Make it clear in the log output that Node.js is not downloaded (except on Windows)

Feedback on irc showed that
```
-- external downloads will be stored/expected in: /home/x/webodf/build/downloads
-- Node.js found: NODEJS_EXECUTABLE-NOTFOUND/0.0.0
CMake Error at CMakeLists.txt:165 (message):
  Node.js is required in version 0.10.5 or later
```
can be misunderstood in that Node.js was tried to be downloaded. Being explicit there should improve DX (developer experience :sake:).