Update JsonFormatException with field name and exception

Author: github-vincent-baret

src/main/scala/scalapb/json4s/JsonFormat.scala

@@ -651,7 +651,7 @@ class Parser private (config: Parser.ParserConfig) {
 
             PMessage(valueMapBuilder.result())
           case _ =>
-            throw new JsonFormatException(s"Expected an object, found ${value}")
+            throw new JsonFormatException(s"Expected an object for ${cmp.scalaDescriptor.fullName}, found ${value}")
         }
     }
   }
@@ -723,11 +723,19 @@ class Parser private (config: Parser.ParserConfig) {
         res.fold[PValue](PEmpty)(PEnum.apply)
       }
       case ScalaType.Message(md) =>
-        fromJsonToPMessage(
-          containerCompanion.messageCompanionForFieldNumber(fd.number),
-          value,
-          false
-        )
+        try {
+          fromJsonToPMessage(
+            containerCompanion.messageCompanionForFieldNumber(fd.number),
+            value,
+            false
+          )
+        } catch {
+          case ex: JsonFormatException =>
+            throw new JsonFormatException(
+              s"Failed parsing field ${fd.name}: ${ex.getMessage}",
+              ex
+            )
+        }
       case st =>
         JsonFormat.parsePrimitive(
           fd.protoType,

src/test/scala/scalapb/json4s/JsonFormatSpec.scala

@@ -456,6 +456,17 @@ class JsonFormatSpec
     )
   }
 
+  "TestProto" should "fail to parse a field of type Message when it's not a json object" in {
+    val jsonFormatException = intercept[JsonFormatException] {
+      new Parser().fromJsonString[MyTest](
+        """{"optMessage": 39}"""
+      )
+    }
+    jsonFormatException.getMessage must be (
+      "Failed parsing field opt_message: Expected an object for jsontest.MyTest, found JInt(39)"
+    )
+  }
+
   "TestProto" should "parse original field names" in {
     new Parser().fromJsonString[MyTest]("""{"opt_enum":1}""") must be(
       MyTest(optEnum = Some(MyEnum.V1))