RFR: 8278595: Provide more information when a pipeline can't be used

[Johan Vos]

This PR provides at least some basic information when a pipeline can't be used. 
There are a number of reasons why a candidate pipeline can't be used, and this PR makes it clear at which point the pipeline fails. 

The changes are only applied in case `prism.verbose` is already set to `true`, so there is no unwanted logging.

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Commit messages:
 - In case prism.verbose=true, this patch provide some basic information

Changes: https://git.openjdk.java.net/jfx/pull/693/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jfx&pr=693&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8278595
  Stats: 14 lines in 2 files changed: 12 ins; 2 del; 0 mod
  Patch: https://git.openjdk.java.net/jfx/pull/693.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx pull/693/head:pull/693

PR: https://git.openjdk.java.net/jfx/pull/693