Imrove error reporting

[mikehaertl]

It seems to be the case that the return value of proc_close() is not really reliable. So sometimes it reports an error, even if everything went fine. We should improve this somehow.

[dreizwo]

For me too. I get exitCode 1 instead of !==0. When changing line 324 in Command.php
from

 if ($this->_exitCode!==0) {
....
}

into

 if ($this->_exitCode==-1) {
....
}

everything works fine

[sukrosono]

as here i think it's related to this. Right now i still can't find reliable way to reproduce my problem, But this capture should show you the basic expectation. I use new PC but i was forgot to install pdftk first, before using php-pdftk.

i was expecting pdftk command not found or something similar. And with that will give usefull error message.

##Update 21-09-2017
As i recall it's run fine on linux, But it fail on windwos. I try to reproduce with travis, It did what i expect. But yesterday i face it again on windows.

[mikehaertl]

Catching the error message has been completely refactored in #41 so this should no longer be an issue.