Better error message when `system` and/or `exec` functions are disabled

[thiagoalessio]

Expected behavior

Present a clear error message to users if environment has disabled system and/or exec builtin PHP functions, and interrupt the execution before attempting to use such disabled functions.

Actual behavior

Right now nothing stops the execution, and it fails much later with a "Command did not produce any output" exception.
Only warnings are shown:

<b>Warning</b>:  system() has been disabled for security reasons in ...
<b>Warning</b>:  exec() has been disabled for security reasons in ...

Steps to reproduce the behavior

1. Disable system and/or exec in php.ini
   https://www.php.net/manual/en/ini.core.php#ini.disable-functions
2. Try to run any TesseractOCR example.

Environment

• Operating system: any
• PHP version: any
• Tesseract version: any
• tesseract-ocr-for-php version: 2.7.4 or lower

[ThomasLandauer]

Maybe you could also add some nice method to check if tesseract is installed at all. When (mis)using $ocr->version() for this, I'm getting 1: if it's not installed - probably some artifact of the exploding that's done at Command::getTesseractVersion()