server: fix crash when any webserver dependency is not installed

[cosven]

Before, when a user enables webserver and does not install sanic, the app just crashes.
Now, log a error message instead.

[coveralls]

Pull Request Test Coverage Report for Build 11700931732

Details

• 0 of 5 (0.0%) changed or added relevant lines in 1 file are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage decreased (-0.009%) to 55.375%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
feeluown/app/server_app.py	0	5	0.0%

Files with Coverage Reduction	New Missed Lines	%
feeluown/app/server_app.py	1	70.59%

Totals
Change from base Build 11546214442:	-0.009%
Covered Lines:	9983
Relevant Lines:	18028

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💛 - Coveralls