Improve logging and error reporting of TagToShortcode task

[maxime-rainville]

Description

The TagToShortCode task doesn't currently provide much feedback if a shortcode is not converted.

If a specific short code conversation fails, the entire task fails.

We should log a warning when an a file we expect should be found can not be found. And we should wrap the function that tries to find expression to converts in each HTMLText value in a try-catch so the task is more resilient.

[sminnee]

This will be easier if the task provides a logger object.

I see that with MigrateFileTask we've done this, but I wonder if adding silverstripe/silverstripe-framework#9183 to 4.5 core would make this kind of stuff slightly less hacky (or at least we have 1 hack, as opposed to a hack-per-BuildTask)