Elaborate Error Message for rpcInternal

[ckeshava]

High Level Overview of Change

Validator operators are often flummoxed by the below error:

It would be helpful if the error message can instruct the users to run the rippled server in another process (./rippled) before executing any of the command-line client commands.

Context of Change

Type of Change

• [ ] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [ ] Refactor (non-breaking change that only restructures code)
• [ ] Performance (increase or change in throughput and/or latency)
• [ ] Tests (you added tests for code that already exists, or your new feature included in this PR)
• [x] Documentation update
• [ ] Chore (no impact to binary, e.g. .gitignore, formatting, dropping support for older tooling)
• [ ] Release

This is an elaboration of an error message.

API Impact

• [ ] Public API: New feature (new methods and/or new fields)
• [ ] Public API: Breaking change (in general, breaking changes should only impact the next api_version)
• [ ] libxrpl change (any change that may affect libxrpl or dependents of libxrpl)
• [ ] Peer protocol change (must be backward compatible or bump the peer protocol version) No change to API. No impact on performance either.

I manually verified (on MacOS) that the new error message is displayed, if rippled server isn't running in the background.

[seelabs]

Needs to be reformatted.