use request body in complete upload, improve exception handling

moto/s3/exceptions.py

@@ -38,3 +38,36 @@ class MissingBucket(BucketError):
             "NoSuchBucket",
             "The specified bucket does not exist",
             *args, **kwargs)
+
+
+class InvalidPartOrder(S3ClientError):
+    code = 400
+
+    def __init__(self, *args, **kwargs):
+        super(InvalidPartOrder, self).__init__(
+            "InvalidPartOrder",
+            ("The list of parts was not in ascending order. The parts "
+             "list must be specified in order by part number."),
+            *args, **kwargs)
+
+
+class InvalidPart(S3ClientError):
+    code = 400
+
+    def __init__(self, *args, **kwargs):
+        super(InvalidPart, self).__init__(
+            "InvalidPart",
+            ("One or more of the specified parts could not be found. "
+             "The part might not have been uploaded, or the specified "
+             "entity tag might not have matched the part's entity tag."),
+            *args, **kwargs)
+
+
+class EntityTooSmall(S3ClientError):
+    code = 400
+
+    def __init__(self, *args, **kwargs):
+        super(EntityTooSmall, self).__init__(
+            "EntityTooSmall",
+            "Your proposed upload is smaller than the minimum allowed object size.",
+            *args, **kwargs)