improve media upload error messages (#2602)

app/src/main/java/com/keylesspalace/tusky/util/ThrowableExtensions.kt

@@ -0,0 +1,26 @@
+package com.keylesspalace.tusky.util
+
+import org.json.JSONException
+import org.json.JSONObject
+import retrofit2.HttpException
+
+/**
+ * checks if this throwable indicates an error causes by a 4xx/5xx server response and
+ * tries to retrieve the error message the server sent
+ * @return the error message, or null if this is no server error or it had no error message
+ */
+fun Throwable.getServerErrorMessage(): String? {
+    if (this is HttpException) {
+        val errorResponse = response()?.errorBody()?.string()
+        return if (!errorResponse.isNullOrBlank()) {
+            try {
+                JSONObject(errorResponse).getString("error")
+            } catch (e: JSONException) {
+                null
+            }
+        } else {
+            null
+        }
+    }
+    return null
+}