RFR: 6833: Tolerate exceptions when loading images

[Alex Ciminian]

This PR fixes the crash reported in
[6833](https://bugs.openjdk.java.net/projects/JMC/issues/JMC-6833?filter=allopenissues) by returning null in case
there's an error reading UI images.

During manual testing I noticed that the icon for Method Profiling can't be loaded on my system and this was causing
the application to crash. I didn't find other resources with this problem.

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Commit messages:
 - 6833: Tolerate exceptions when loading images

Changes: https://git.openjdk.java.net/jmc/pull/86/files
 Webrev: https://webrevs.openjdk.java.net/jmc/86/webrev.00
  Issue: https://bugs.openjdk.java.net/browse/JMC-6833
  Stats: 9 lines in 1 file changed: 8 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jmc/pull/86.diff
  Fetch: git fetch https://git.openjdk.java.net/jmc pull/86/head:pull/86

PR: https://git.openjdk.java.net/jmc/pull/86