RFR: 8046: Detect failing agent builds on Windows

[Ralf Schmelter]

When running build.bat --packageAgent, the build.bat doesn't detect if the maven call fails, since the ERRORLEVEL variable is overridden by the "cd .." call before it is checked. Additionally the format of the if/else construct is wrong (uses curly braces in the else part).

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Commit messages:
 - Detect failing agent builds

Changes: https://git.openjdk.org/jmc/pull/472/files
 Webrev: https://webrevs.openjdk.org/?repo=jmc&pr=472&range=00
  Issue: https://bugs.openjdk.org/browse/JMC-8046
  Stats: 4 lines in 1 file changed: 1 ins; 1 del; 2 mod
  Patch: https://git.openjdk.org/jmc/pull/472.diff
  Fetch: git fetch https://git.openjdk.org/jmc pull/472/head:pull/472

PR: https://git.openjdk.org/jmc/pull/472