RFR: JDK-8287337: SnippetUtils should throw exceptions if snippets not found

[Jonathan Gibbons]

Please review a simple change for `SnippetUtils` to throw a checked exception if a snippet cannot be found.

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Commit messages:
 - JDK-8287337: SnippetUtils should throw exceptions if snippets not found

Changes: https://git.openjdk.java.net/jdk/pull/8892/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=8892&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8287337
  Stats: 41 lines in 2 files changed: 31 ins; 6 del; 4 mod
  Patch: https://git.openjdk.java.net/jdk/pull/8892.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/8892/head:pull/8892

PR: https://git.openjdk.java.net/jdk/pull/8892