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Updates RangeSize error message and tests for IPv6.
**What this PR does / why we need it**:
Updates the RangeSize function's error message and tests for IPv6. Converts RangeSize unit test to a table test and tests for success and failure cases. This is needed to support IPv6. Previously, it was unclear whether RangeSize supported IPv6 CIDRs. These updates make IPv6 support explicit.
**Which issue this PR fixes**
Partially fixes Issue #1443
**Special notes for your reviewer**:
/area ipv6
**Release note**:
```NONE
```
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