Throw error if manual color ids are not integers

[jnalanko]

The manual color file option expects a file with one integer per line. If the line something else than a number, Themisto will not crash but will instead produce garbage results. We should crash instead with an error message.

Checking for invalid input may lead to worse performance though, but the color file should be quite small typically compared to the full sequence data, so this error check is worth the cost.