RFR: 8276694: Pattern trailing unescaped backslash causes internal error

[Masanori Yano]

Could you please review the 8276694 bug fixes?

A message specific for this exception should be printed instead of an internal error. This fix adds a new check to output an appropriate exception message when the regular expression ends with an unescaped backslash. This fix also checks the position of the cursor to rule out other syntax errors at the middle position of the regular expression.

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Commit messages:
 - 8276694: Pattern trailing unescaped backslash causes internal error

Changes: https://git.openjdk.java.net/jdk/pull/6891/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=6891&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8276694
  Stats: 12 lines in 2 files changed: 11 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6891.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6891/head:pull/6891

PR: https://git.openjdk.java.net/jdk/pull/6891