[Heartbeat] Improve body mismatch error

[shahzad31]

Describe the enhancement:
When user includes body match check and body doesn't match, heartbeat generated body mismatch error like this

i think error generated can be more explicit saying, expected "body" doesn't match with response body. It will give user more information in uptime app without digging deeper into heartbeat.

[elasticmachine]

Pinging @elastic/uptime (Team:Uptime)

[botelastic]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[botelastic]

Hi!
We just realized that we haven't looked into this issue in a while. We're sorry!

We're labeling this issue as Stale to make it hit our filters and make sure we get back to it as soon as possible. In the meantime, it'd be extremely helpful if you could take a look at it as well and confirm its relevance. A simple comment with a nice emoji will be enough :+1.
Thank you for your contribution!