Make Photometadata class more robust

[jeremypw]

May fix some issues with loading corrupt metadata.

• Replaces an assert statement with throwing errors
• Replaces deprecated GExiv2 methods with new error throwing ones and handle errors where necessary

[jeremypw]

The probable cause for reported crashes has now been found and fixed (see #785 ) so the changes here are mainly cosmetic - there is no evidence at the moment that they give rise to crashes. As there are no test defined in the codebase any regressions will have to be found by user testing I guess 😞 The codebase is littered with myriad functions that return null but in practice seems fairly stable so whether it is worth the work of updating it is debatable.

[jeremypw]

If desired I can separate smaller PRs for individual fixes that seem more worthwhile than others.