code: private final ByteVec getUploadedMojo(final Key<Frame> key) throws IllegalArgumentException {
          Objects.requireNonNull(key); // Nicer null pointer exception in case null key is accidentally provided
  
          Frame mojoFrame = key.get();
          if (mojoFrame.numCols() > 1)
              throw new IllegalArgumentException(String.format("Given MOJO frame with key '%s' should contain only 1 column with MOJO bytes. More columns found. Incorrect key provided ?", key));
          ByteVec mojoData = (ByteVec) mojoFrame.anyVec();
  
          if (mojoData.length() < 1)
              throw new IllegalArgumentException(String.format("Given MOJO frame with key '%s' is empty (0 bytes). Please provide a non-empty MOJO file.", key));
  
          return mojoData;
      }
docs: Retrieves pre-uploaded MOJO archive and performs basic verifications, if present.
  
  @param key Key to MOJO bytes in DKV
  @return An instance of {@link ByteVec} containing the bytes of an uploaded MOJO, if present. Or exception. Never returns null.
  @throws IllegalArgumentException In case the supplied key is invalid (MOJO missing, empty key etc.)