* remove popups, sidepanes, helppanes.
* verify file existence before opening.
* refine error/source windows:
* find window showing the error source buffer,
split a new one if none are found.
* if current window is showing error source buffer,
find file in next window, split a new one if none are found.
* center cursor after going to error.
* show error line on window's first line.
* center cursor after going to error.
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