Improve DefaultDubboFallback implementation(apache-dubbo-adapter).

[wuwen5]

Describe what this PR does / why we need it

The caller cannot determine the type of runtime exception thrown.
I suggest the same way as sentinel-dubbo-adapter(dubbo-2.6.x).

Does this pull request fix one issue?

#1252

[sczyh30]

Hi, thanks for contributing. The exception type has been carried in the message of the wrapped RuntimeException. SentinelRpcException is not recommended any more.

[wuwen5]

SentinelRpcException is a subclass of RuntimeException so callers can capture and judge it easily, while RuntimeException (ex.toRuntimeException()) can only be captured and judged by error message, which is very unfriendly.

try {...
} catch (RuntimeException e) {
 if (e.getMessage().contains("SentinelBlockException")) {
                //...
  }
}

try {
} catch (SentinelRpcException e) {
  // is friendly
}