Fix: Plugin configuration validation during dry-run

[ngandrass]

This PR addresses #10 and improves the plugin config validation in general:

• Improve plugin config validation routine
• Display reason for failed plugin config validation on dry-run page
• Fix missing language string on dry-run page if plugin config is invalid

[coveralls]

Pull Request Test Coverage Report for Build 19130599645

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 9 unchanged lines in 1 file lost coverage.
• Overall coverage decreased (-1.0%) to 96.691%

---

Files with Coverage Reduction	New Missed Lines	%
moodle/admin/tool/userautodelete/classes/manager.php	9	95.96%

Totals
Change from base Build 18845688907:	-1.0%
Covered Lines:	263
Relevant Lines:	272

---

💛 - Coveralls