RFR(S) 8227435: Perf::attach() should not throw a java.lang.Exception

[Schmelter, Ralf]

Please review this small change. It changes the exception which will be thrown when the perf file has not yet the correct size. Instead of throwing the (not declared) java.lang.Exception, we will now throw java.io.IOException, which is expected by the calling code.

webrev: http://cr.openjdk.java.net/~rschmelter/webrevs/8227435/webrev.0/
bugreport: https://bugs.openjdk.java.net/browse/JDK-8227435

Best regards,
Ralf
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