Show parse issues in GUI

[janslow]

At the moment, if an invalid CSV file is parsed, the user must check the console for the reasons why.

A summary of the errors should be displayed in a dialog box instead.

[janslow]

We should add a ParseIssue class which contains the severity, location and description of a issue which is discovered when parsing.

When each section of the CSV is parsed, it should either return a valid result (i.e., a cue or group of cues) or a set of ParseIssues.

The severities should be as follows:

• FATAL - Not a valid CSV file, so couldn't read any rows (such as missing a closing quote mark, couldn't read file or missing columns)
• ERROR - Unable to parse a cue or group of cues (such as ommitting the ID field for a row, couldn't extract cue from parts of a string)
• WARNING - Parsed everything fine, but this may cause issues elsewhere (such as invalid patch numbers, duplicate IDs)