RFR: JDK-8287337: SnippetUtils should throw exceptions if snippets not found [v2]

[Jonathan Gibbons]

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

Jonathan Gibbons has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains three additional commits since the last revision:

 - address review feedback
 - Merge remote-tracking branch 'upstream/master' into 8287337.snippetutils
 - JDK-8287337: SnippetUtils should throw exceptions if snippets not found

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Changes:
  - all: https://git.openjdk.java.net/jdk/pull/8892/files
  - new: https://git.openjdk.java.net/jdk/pull/8892/files/1d40bb1d..6d0f55fd

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=8892&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=8892&range=00-01

  Stats: 42320 lines in 364 files changed: 19210 ins; 17720 del; 5390 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