Perform early check if image upload may be possible. Fixes #82

[derwok]

In case of problems:

• Give error log feedback with solution hints
• Return error code 21 to shell
• Terminate script

Fixes #82

[andig]

Ich denke die prerequisites gehören ins runcommand, nicht in die functions da auch die Fehlertexte ausschließlich vom command abhängen?

[derwok]

Moved error texts to run command. I was uncertain what belongs to "function.php" and what not...