Use custom exception in CharBuffer

[tomas-langer]

The reactive/media/common/src/main/java/io/helidon/reactive/media/common/CharBuffer.java is using OutOfMemoryError as an exception.
This should not be done - OOM is reserved to be thrown by the JVM when it runs out of memory, not by us when we are "afraid of running out of memory".

This should be changed to a custom runtime exception (not an Error)