documentation: description of --error-exit-code is not entirely true

[Reissner]

From the documentation, Section 3.2 we read:

--error-exit-code={true,false} By default, pythontex.py returns an
exit code of 1 if there were any errors, and an exit code of 0 otherwise.

In fact, there are kind of 'fatal errors' leading to an exit code 1, 2, or a message immediately.
On the other hand, there are non-fatal errors which are counted.
The abovementioned option refers to the non-fatal errors only.
This shall be reflected in the documentation.

In the text above, I think one should state that true ist the default, this would be clearer.