* Added a report list to operator, to which they can report errors and
warnings. When the operator ends, it will display them with a popup. For
python these should become exceptions when calling operators.
* Added a function to make a popup menu from a report list.
* Also added a utility function to prepend a string before the reports to
indicate what they relates to. Also made the report functions used
BLI_dynstr to simplify the code.
* Made file reading and writing report errors to the user again using this
system, also replacing the left over uncommented bad level error() calls.
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