Problem:    Code is indented more than necessary.
Solution:   Use an early return where it makes sense. (Yegappan Lakshmanan,
            closes #11805)

Problem:    Error messages are spread out.
Solution:   Move the last error messages to errors.h.

Problem:    Error messages are spread out.
Solution:   Move more error messages to errors.h.

Problem:    Error messages are spread out.
Solution:   Move more error messages to errors.h.

Problem:    Error messages are spread out.
Solution:   Move more error messages to errors.h.

Problem:    Error messages are spread out.
Solution:   Move more errors to errors.h.

Problem:    Various spelling mistakes in comments.
Solution:   Fix the mistakes. (Dominique Pellé, closes #9416)

Problem:    Dynamic library load error does not mention why it failed.
Solution:   Add the error message. (Martin Tournoij, closes #8621)

Problem:    Python 3: non-utf8 character cannot be handled.
Solution:   Change the string decode. (Björn Linse, closes #1053)

Problem:    Compiler warnings when using Python.
Solution:   Adjust PyCFunction to also have the second argument.  Use "int"
            return type for some functions.  Insert "(void *)" to get rid of
            the remaining warnings.