Petascope - return better error message when trimming over non-existing coefficients of an irregular axis

[Bang Pham Huu]

For example, this query:

for c in (test_irr_cube_2)

return encode(c[ansi("2008-01-06":"2008-01-07")], "jpeg")

on coverage test_irr_cube_2 with the coefficients:

<gmlrgrid:coefficients>"2008-01-01T02:01:20.000Z" "2008-01-03T23:59:55.000Z" 
"2008-01-05T01:58:30.000Z" "2008-01-08T00:02:58.000Z"</gmlrgrid:coefficients>

which returns something misunderstanding:

Reason: Input lower bound '4.91574074074074074074074074074074074074074074074074' is greater than upper bound '5.91574074074074074074074074074074074074074074074074' in direct positions list of irregular axis 'ansi'.