8361115: javax/swing/JComboBox/bug4276920.java unnecessarily throws Error instead of RuntimeException

[manukumarvs]

Issue:
javax/swing/JComboBox/bug4276920.java unnecessarily throws Error instead of RuntimeException whenever a test failure occurs.
This causes unnecessary extra attention in CI systems and triggers task failures as throwing an Error will write the error message to System.err and the CI system will detect it as an 'Error' and fails the task eventually.
But this is just a normal test failure only and just need to throw 'RuntimeException' and no need to throw an 'Error'.

Fix:
Replace 'Error' with 'RuntimeException'

Testing:
Tested using mach5 in all the available platforms.
Tested manually the negative (failure) condition also and verified that its throwing 'RuntimeException' in cases of failure.

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Issue

• JDK-8361115: javax/swing/JComboBox/bug4276920.java unnecessarily throws Error instead of RuntimeException (Bug - P4)

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