Exit if exe is not found

[Nico01]

The program keep running even if not executable is found, this lead to a segfault.

[JayFoxRox]

Thanks for your PR. While the PR is fine as is, I have some criticism / constructive feedback:

• You should not work on your master branch, but instead create a new branch per feature
• Please always write a small description for your PR (in the first post) which states the motivation
• Please start your commit messages with a capital letter

This PR also raises some interesting questions about OpenSWE1R coding style:

• I'm not sure about the use of EXIT_SUCCESS and EXIT_FAILURE. Personally I'd rather have seen exit(1) as we don't use EXIT_SUCCESS and mixing them feels like a bad idea
• Not sure about the use of exit() in main, as return would do the same
• Shall we print errors caller or callee side? This PR kind of splits it: errno / perror in callee, but another printf() in caller.

I don't know why Travis was not ran for this PR. Travis received the PR notification, and tried to run the build 9 times, but all came back with "Nothing here" status. I'll manually try to trigger a build (on my fork) to see if this is macOS and Windows compatible. (Works fine: AppVeyor, Travis)

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Please fixup / squash (and force push) your commits if you decide to change anything.

[Nico01]

I suggest to use EXIT_SUCCESS/EXIT_FAILURE instead of 0/1 because improve readability and the behaviour is the same. As for error printing, check the return value and handle the result from the caller maybe better for keep the function cleaner. A coding style guideline would be nice and the code need to be formatted somehow (clang-format ?). inb4 my english suck

[JayFoxRox]

I've made some modifications to this PR:

• Do a callee side error check only for things which could not be moved easily (we don't want to use errno to pass error codes around for example, so perror should remain in the callee)
• Do a caller side error check as it would be necessary for most std functions or C++ exceptions
• Used exit() so the code can later be moved outside of main() without changes.
• Added a new commit to use the EXIT_ constants as you originally intended.

I'll merge this PR after introducing a CLA (which you'll have to accept); and disclaimers to the README. More about this here.

Meanwhile, please review if my changes reflect your original intention.

clang-format is also planned for the future, it will be ran on Travis CI and report any style issues. I just don't have the time to work on these things right now: input, graphics, audio and multiplayer are more important. Also, your english is fine :)

[JayFoxRox]

@Nico01 the CLA has been finalized. Please sign it under this link.

(I'd appreciate if you sign it so we can test the ecosystem, but I'll still merge this anyway, simply because it's such a small change, so we are fine legally anyway)

[Nico01]

Done :)