Only unlink existing files

[tobias-kuendig]

If for any reason the index file does not exist the delete action should be considered successful.

Whithout this check the deletion of non-existent DB entries fails with an ErrorException.

[coveralls]

Pull Request Test Coverage Report for Build 120

• 1 of 2 (50.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.2%) to 92.788%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/Filesystem.php	1	2	50.0%

Totals
Change from base Build 118:	-0.2%
Covered Lines:	476
Relevant Lines:	513

---

💛 - Coveralls

[timothymarois]

Perfect. I like it. I'll push this in the next release.