simplecpp silently ignores when an explicitly included file does not exist

[versat]

This has been reported in the Cppcheck forum:
https://sourceforge.net/p/cppcheck/discussion/general/thread/f20832cc83/

If simplecpp is called with -include=<file>, and the file does not exist, there is no error message. simplecpp just ignores the file. This could be very confusing for users since the real issue thus can be hidden for a long time.
At simplecpp:2426 a not existing file is simply ignored.
GCC aborts with a fatal error in the same case.
To not break the compatibility, simplecpp could be changed to print some warning so the user knows that the file is not included but otherwise behave as before.
I suggest the same handling for include files that exist but can not be opened for some reason.

[versat]

@danmar Would it be ok to print an error message to stderr (or stdout?) in that case to inform the user? I could create a PR for that.

[danmar]

@versat I do not think a library should print to stderr or stdout directly. Please generate an OutputListItem error message so the clients can handle this.
Imho it's fine if this is a fatal error. This is very likely something that the user really wants to fix.