Improve error reporting in task setting editor

[effa]

Report specific errors on specific lines, using annotations and markers which the AceEditor offers. The most clean way is probably to implement custom InvalidTaskSettingError carrying the information about specific problem. Errors to be handled and reported:

• wrong world shape (each row should have same number of columns)
• at least 1 row and 1 column
• missing background
• invalid background (and tell which are valid)
• invalid object (and tell which are valid)
• missing spaceship / more than 1 spaceship
• spaceship not on the bottom line
• top line should be blue
• other blue fields not allowed