RFR: 8304290: Some JNI calls made without checking exceptions in media

[Alexander Matveev]

- Added missing exception checks for JNI calls.
 - Improved JNI error checking by checking for both exception and return value.
 - Minor code clean up.

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Commit messages:
 - 8304290: Some JNI calls made without checking exceptions in media

Changes: https://git.openjdk.org/jfx/pull/1094/files
 Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=1094&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8304290
  Stats: 278 lines in 15 files changed: 80 ins; 92 del; 106 mod
  Patch: https://git.openjdk.org/jfx/pull/1094.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1094/head:pull/1094

PR: https://git.openjdk.org/jfx/pull/1094