Don't mention "error" in error messages

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.

mesonbuild/interpreter.py

@@ -1960,7 +1960,7 @@ to directly access options of other subprojects.''')
     @noKwargs
     def func_error(self, node, args, kwargs):
         self.validate_arguments(args, 1, [str])
-        raise InterpreterException('Error encountered: ' + args[0])
+        raise InterpreterException('Problem encountered: ' + args[0])
 
     def detect_compilers(self, lang, need_cross_compiler):
         cross_comp = None