Improved error message rendering for nbb and nbb repl

[andersmurphy]

Currently the error messages in nbb and nbb repl are returned as strings eg:

"#error {:message \"Could not resolve symbol: createUmi\", :data {:type :sci/error, :line 2, :column 7, :message \"Could not resolve symbol: createUmi\", :sci.impl/callstack #object[cljs.core.Volatile {:val ({:line 1, :column 1, :ns #object[Is user], :file nil} {:line 2, :column 3, :ns #object[Is user], :file nil} {:line 2, :column 3, :ns #object[Is user], :file nil, :sci.impl/f-meta {:ns #object[Is clojure.core], :macro true, :sci/built-in true, :name ->, :arglists ([x & forms]), :doc \"Threads the expr through the forms. Inserts x as the\\n second item in the first form, making a list of it if it is not a\\n list already. If there are more forms, inserts the first form as the\\n second item in second form, etc.\"}} {:line 3, :column 5, :ns #object[Is user], :file nil} {:line 2, :column 7, :ns #object[Is user], :file nil})}], :file nil, :phase \"analysis\"}, :cause #error {:message \"Could not resolve symbol: createUmi\", :data {:type :sci/error, :line 1, :column 1, :file nil, :phase \"analysis\"}}}"

This makes them hard to read. Returning these errors as a map rather than a string and pretty printing them would make them much easier to parse (this would also remove the nested string quotes etc).

Related to this, is there a simple way to add a pretty printer for output (this comes up in environments where nrepl/socket relp are not available)?

Thanks.