Improve the exception message in the UrlMatcher.
minor #11767 Improve the exception message of UrlMatcher (dawehner)

This PR was merged into the 2.6-dev branch.

Discussion
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Improve the exception message of UrlMatcher

The current exception message is not optimal, some more context always helps.

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| License       | MIT

Commits
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ee1197d Improve the exception message in the UrlMatcher.