Ensure checker outputs say "you put X but we expected Y" wherever possible.

[agstephens]

This could be easily improved:

--------------------------------------------------------------------------------
                         IOOS Compliance Checker Report
                   product_pm-concentration_land_checks:v2.0

--------------------------------------------------------------------------------
                               Corrective Actions
uea-fidas200E-1_ral_29001225_pm-concentration_v0.1.nc has 38 potential issues


                                 High Priority
--------------------------------------------------------------------------------
Global attribute: Conventions
* Required 'Conventions' global attribute value does not match regex 'CF\-1\.6,\ NCAS\-AMF\-2\.0\.0'.

Global attribute: platform
* Required 'platform' global attribute value is invalid.

Global attribute: platform_altitude
* Required 'platform_altitude' global attribute value does not match regex '-?\d+(\.\d+)? m'.

[agstephens]

@gapintheclouds: let's do:

• In a new branch: update the template messages in the compliance-check-lib register of check classes wherever we can make them more readable.
• Create a PR on Github
• Get review from GP/WG