Stacktrce is logged on invalid imput to migration module

[kofemann]

If invalid input is provided, then the migration endpoint logs a stacktrace instead of appropriate error log message:

18 Dec 2025 11:32:50 (Frontend-S) [] Failed to parse JSON "{
  "sourcePool": "pool-a",
  "targetPools": ["pool-b"],
  "exclude": ["pool_res1]
}": Illegal unquoted character ((CTRL-CHAR, code 10)): has to be escaped using backslash to be included in string value
 at [Source: REDACTED (`StreamReadFeature.INCLUDE_SOURCE_IN_LOCATION` disabled); line: 4, column: 26]
18 Dec 2025 11:32:50 (Frontend-S) [] Uncaught exception in thread jetty-164
org.json.JSONException: Unterminated string at 83 [character 0 line 5]
	at org.json.JSONTokener.syntaxError(JSONTokener.java:503)
	at org.json.JSONTokener.nextString(JSONTokener.java:302)
	at org.json.JSONTokener.nextSimpleValue(JSONTokener.java:430)
	at org.json.JSONTokener.nextValue(JSONTokener.java:421)
	at org.json.JSONArray.<init>(JSONArray.java:105)
	at org.json.JSONTokener.nextValue(JSONTokener.java:416)
	at org.json.JSONObject.<init>(JSONObject.java:237)
	at org.json.JSONObject.<init>(JSONObject.java:402)
	at org.dcache.restful.resources.migration.MigrationResources.submitMigrationCopy(MigrationResources.java:123)
	at java.base/jdk.internal.reflect.DirectMethodHandleAccessor.invoke(Unknown Source)