feat+fix+test: exec:default(portexec) with better errors and checks (different cases)

[rNoz]

After this #185, now that we have a test detecting the current value returned by that function, let's modify functionality of exec:default(portexe), providing better errors and "" under wrong conditions.

4 new tests will show its behavior under different scenarios.

===> Verifying dependencies...
===> Analyzing applications...
===> Compiling erlexec
===> Performing EUnit tests...
....................
==> Test Concurrency: 900
......
Finished in 15.443 seconds
26 tests, 0 failures

Remaining bit:

• I would like to refactor setup/teardown of tests (a private helper function to rename/backup priv dir).
• I manually format these lines but let me know with further suggestions :)

Again, thanks for this library!

Updating PR showing how I am testing it from an Elixir release. Warning message is clarifying it:

[saleyn]

@rNoz this requires a rebase on the other merged PR

[rNoz]

@saleyn thanks! rebased :)

In a few days I could invest a few more time to refactor the setup sections, but test/code-wise is complete and fully checked. We can wait or I can provide the refactors as another PR (along with another minor proposal).

[rNoz]

@saleyn ready, just refactored :)