Fix context.next() to throw Route not found instead of TypeError

[frenzzy]

Types of changes

• [x] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• [x] My code follows the code style of this project.
• [ ] My change requires a change to the documentation.
• [ ] I have updated the documentation accordingly.
• [x] I have read the CONTRIBUTING document.
• [x] I have added tests to cover my changes.
• [x] All new and existing tests passed.

Fixes #168

[codecov-io]

Codecov Report

Merging #169 into master will not change coverage. The diff coverage is 100%.

@@          Coverage Diff          @@
##           master   #169   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files           7      7           
  Lines         181    181           
  Branches       53     53           
=====================================
  Hits          181    181

Impacted Files	Coverage Δ
src/UniversalRouterSync.js	100% <100%> (ø)	:arrow_up:
src/UniversalRouter.js	100% <100%> (ø)	:arrow_up:

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