RFR: defpath: normalize error format

[Erik Helin]

Hi all,

please review this small patch that makes `git defpath` use the same error message as `git jcheck` and `git webrev`
when not finding a local repository.

Testing:
- [x] Manual testing on Linux x64

Thanks,
Erik

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Commit messages:
 - defpath: normalize error format

Changes: https://git.openjdk.java.net/skara/pull/833/files
 Webrev: https://webrevs.openjdk.java.net/?repo=skara&pr=833&range=00
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/skara/pull/833.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/833/head:pull/833

PR: https://git.openjdk.java.net/skara/pull/833