Non existing source files are silenctly ignored

[Torsten Rupp]

It seems non existing files are silently ignored if a least one source file is found.

Reproduce:

touch a.cpp
cppcheck a.cpp missing.cpp
echo $?

Result: 0

Expected behavior: some error message like
cppcheck: error: could not find 'missing.cpp'

However this produce an error message as expected:

cppcheck missing.cpp
cppcheck: error: could not find or open any of the paths given.

Thus if at least one source file is found all not existing other source files are silently ignored.

[Aaron Danen]

in cli/filelister.cpp:addFiles2 there is

    struct stat file_stat;
    if (stat(path.c_str(), &file_stat) == -1)
        return ""; // TODO: return error?

if we returned an error here we could help users who are in this situation. I don't know what situation there would be for a user to specify the analysis of a nonexistent file. The most common thing would probably be people mistyping the name of a file or something similar, in which case proceeding as usual is probably an annoying behavior.