improve handling of non-finite floating point values (#490)

* improve HTTP error reporting

* generate standards-compatible JSON

* add --nan-to-num work-around for non-finite floating point values

* lint

* update tests

* correctly set Infinities to min/max

* REAMDE update for --nan-to-num

* define constant for repetitive warning message

* clarify where NaN errors will occure

server/app/util/constants.py

@@ -25,3 +25,6 @@ class Axis(AugmentedEnum):
 class DiffExpMode(AugmentedEnum):
     TOP_N = "topN"
     VAR_FILTER = "varFilter"
+
+
+JSON_NaN_to_num_warning_msg = "JSON encoding failure - suggest trying --nan-to-num command line option"