Handle low post_max_size

[mbollmann]

Originally reported by: Marcel Bollmann (Bitbucket: mbollmann, GitHub: mbollmann)

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If post_max_size is too low, CorA can fail when trying to import large texts. This is natural.

However, since this causes the $_POST variable to just be empty, the server will answer with "Unknown request", which is not helpful. The PHP documentation recommends using a separate $_GET variable to check for this.

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• Bitbucket: https://bitbucket.org/mbollmann/cora/issue/64