Turns out

error.SuiteScriptError instanceof Error

is

false

First time this has bitten me Thought I'd share: while debugging I found this snippet from the source code of

N/error

//inheritance TODO this is not working for Error, SuiteScriptError is not an instanceof Error
var debuggable = nsobject.getNewInstance();
var err = Object.create(Error.prototype);
debuggable.prototype = err;
SuiteScriptError.prototype = debuggable;
SuiteScriptError.prototype.constructor = SuiteScriptError;