RFR: Make "git debug mlrules" tolerate missing files, and clarify help

[Magnus Ihse Bursie]

The "mlrules" debug command crashed if given non-existing files.

Also clarify the difference between files to test for, and path to the config file.

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Commit messages:
 - Make "git debug mlrules" tolerate missing files, and clarify help

Changes: https://git.openjdk.java.net/skara/pull/1093/files
 Webrev: https://webrevs.openjdk.java.net/?repo=skara&pr=1093&range=00
  Stats: 12 lines in 1 file changed: 6 ins; 0 del; 6 mod
  Patch: https://git.openjdk.java.net/skara/pull/1093.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/1093/head:pull/1093

PR: https://git.openjdk.java.net/skara/pull/1093