RFR: 7903020: Command line error: Incorrect output filename - can't write

[Leonid Kuskov]

This is the fix for https://bugs.openjdk.java.net/browse/CODETOOLS-7903020
The fix extends checking of files as existing as non-existing

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Commit messages:
 - 7903020: Command line error: Incorrect output filename - can't write

Changes: https://git.openjdk.java.net/jcov/pull/18/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jcov&pr=18&range=00
  Issue: https://bugs.openjdk.java.net/browse/CODETOOLS-7903020
  Stats: 21 lines in 2 files changed: 14 ins; 0 del; 7 mod
  Patch: https://git.openjdk.java.net/jcov/pull/18.diff
  Fetch: git fetch https://git.openjdk.java.net/jcov pull/18/head:pull/18

PR: https://git.openjdk.java.net/jcov/pull/18