update messaging when file not found

[coveralls]

Pull Request Test Coverage Report for Build f455c371-f435-40ad-a09b-1a9558fb256a

• 11 of 12 (91.67%) changed or added relevant lines in 3 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 64.643%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
tests/utils/mock_server.py	5	6	83.33%
<!--	Total:	11	12	91.67%	-->

Totals
Change from base Build ef104a7f-b482-43db-8db1-8a6adb2903c6:	0%
Covered Lines:	13449
Relevant Lines:	20805

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💛 - Coveralls

[KyleGoyette]

Adding a test, will follow up about a better check which can be performed.

[KyleGoyette]

Added a test, updated the mock server to consider a nonexistent file: nofile.h5