Confusing error message in ModelSelectorSummary#evalMetFromJson

[gerashegalov]

Describe the bug
def error produces a confusing string by calling mkString on a string json.mkString(",")
Moreover it swallows the root cause exceptions needed for debugging and fixing.

To Reproduce
inject corrupt metric json

Expected behavior
original text leading to the serde exception, and the entire cause chain

Logs or screenshots

java.lang.IllegalArgumentException: Could not extract metrics of type class com.salesforce.op.evaluators.MultiMetrics from {,",m,e,t,r,i,c,s,",:,{,",m,u,l,t,i,c,l,a,s,s, ,e,v,a,l,u,a,t,i,o,n, 
...