Added slighly more meaningful messages to a few matchers

[bhjoellund]

Hi!

I was experiencing a few weird error messages, like

Error Message:
 FsUnit.Xunit+MatchException : Exception of type 'FsUnit.Xunit+MatchException' was thrown.
Expected: 1
Actual:   was 1

which does not make a lot of sense, so I added some slightly more meaningful messages to those matchers.

[sergey-tihon]

Thank you @mofibrian !
Could you please use %A in formatters?

sprintf "Greater than %A" x

instead of

sprintf "Greater than %s" (string x)

[bhjoellund]

Hey @sergey-tihon

Absolutely, I'll update the PR.

[sergey-tihon]

Thank you!

[bhjoellund]

You're welcome :-)