RFR: 8282220: contentType should not be a PKCS7's member

[John Jiang]

`contentType` should be a local variable because it is only used in method `parse(DerInputStream derin, boolean oldStyle)`.
This change also improves some exception catch blocks.

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Commit messages:
 - 8282220: contentType should not be a PKCS7's member

Changes: https://git.openjdk.java.net/jdk/pull/7568/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=7568&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8282220
  Stats: 19 lines in 1 file changed: 0 ins; 14 del; 5 mod
  Patch: https://git.openjdk.java.net/jdk/pull/7568.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/7568/head:pull/7568

PR: https://git.openjdk.java.net/jdk/pull/7568