Do not require asp for AUR download

[yuyichao]

Also include a reason in the error message when asp is required but missing.

Fix #485

Tested that -G <aur package> works without asp installed.
-G <official repo package> prints an error without asp and works with it installed.
Other operations like -Syuo still works.

[coveralls]

Pull Request Test Coverage Report for Build 1800

• 5 of 7 (71.43%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.04%) to 87.66%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
pikaur/main.py	5	7	71.43%

Totals
Change from base Build 1798:	-0.04%
Covered Lines:	3005
Relevant Lines:	3428

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💛 - Coveralls

[actionless]

fixed in 9f6586d