Previously, Meson was showing a subproject being downloaded after later
claiming it doesn't exist.
This patch shows the actual error to clarify why the given subproject
can not be used.

Starting with VS 2017 if the output of any command run by VS contains
the word Error it will interpret that as a fatal error, even if the exit
error code is zero.
This messes up the unit tests on VS 2017, because we sometimes want to
deliberately ignore error messages.
Change "Error" to "Problem" to mitigate this issue until a more
permanent solution is found.