Missing variable in freshness check should throw an error or at least warning

[vijaykiran]

running this without scan_execution_date being available as a variable

checks for fact_internet_sales:
  - freshness using ship_date with scan_execution_date < 12h45

it uses current scan run time instead of complaining about missing variable

[m1n0]

Linked PR removes the fallback to current time behaviour and fails the check early.

$ cat checks.yml
checks for sodatest_customers_b7580920:
  - freshness using ts with scan_execution_date < 1d

$ soda scan -d postgres -c configuration.yml checks.yml
Soda Core 3.0.0b9
Could not parse variable scan_execution_date as a timestamp: variable not found
Scan summary:
1/1 check FAILED:
    sodatest_customers_b7580920 in postgres
      freshness using ts with scan_execution_date < 1d [FAILED]
1 errors.
Oops! 1 failures. 0 warnings. 1 error. 0 pass.
ERRORS:
Could not parse variable scan_execution_date as a timestamp: variable not found