[Moodle 4.1] Several uses of deprecated print_error() function

[danowar2k]

Moodle Version in use
Moodle 4.1.13

** Plugin Version **
PDF Annotator v1.5 release 4 (but irrelevant, code is still present in master)

Describe the bug
Moodle's print_error() function is deprecated since Moodle 4.1, see https://tracker.moodle.org/browse/MDL-71062 (and is removed in Moodle 4.5)

There are several spots in the code that still use this function: https://github.com/search?q=repo%3Arwthmoodle%2Fmoodle-mod_pdfannotator%20print_error(&type=code

They should be replaced with throwing exceptions.

[sharpchi]

This patch should resolve this issue

P0026-mod_pdfannotator_deprecated_print_error.txt

[Amrita1991]

Thanks @sharpchi we will be incorporating this in the next release