Remove panic when method not found in the tree

[tadvi]

Replaced panic with HandleNotFound in router.go line 257.

[coveralls]

Pull Request Test Coverage Report for Build 190

• 2 of 2 (100.0%) changed or added relevant lines in 1 file are covered.
• 2 unchanged lines in 1 file lost coverage.
• Overall coverage decreased (-0.7%) to 99.324%

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Files with Coverage Reduction	New Missed Lines	%
router.go	2	99.05%

Totals
Change from base Build 183:	-0.7%
Covered Lines:	294
Relevant Lines:	296

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💛 - Coveralls