add ex info to reject string

[sam-pitch]

currently pptxgenjs swallows the exception when an image experiences an error during fetch. We should forward this so we can get some more helpful info in bugsnag.

[linear]

SWI-405 Bugsnag: Critical about images which couldn't be loaded during file write.

| ERROR! Unable to load image (image.onerror) |

https://app.bugsnag.com/pitch-software-gmbh/app/errors/63888025190c81000a061681?filters[event.since]=7d&filters[app.release_stage]=stable&filters[event.message]=[pptx-exporter]

| ERROR! Unable to load image (xhr.onerror) |

https://app.bugsnag.com/pitch-software-gmbh/app/errors/6388923d437daa0009ed801d?filters[event.since]=7d&filters[app.release_stage]=stable&filters[event.message]=[pptx-exporter]

We've seen this errors a few time. Our speculation is that the user is attempting export with a shaky internet connection which fails to fetch the image from network.

We could try to put some retries in PptxGenJS so that it attempts the write more persistently. If the user's connection is mostly fine though, and they try again. Hopefully they eventually experience a successful export.