update error message for unspecified exceptions

- ask user to report unexpected errors, which usually indicate
  extractor failure
- handle OSErrors separately (permissions, disk full, etc)
- revert 30eef52

gallery_dl/extractor/jaiminisbox.py

@@ -17,7 +17,6 @@ class JaiminisboxChapterExtractor(foolslide.FoolslideChapterExtractor):
     pattern = foolslide.chapter_pattern(r"(?:www\.)?jaiminisbox\.com/reader")
     test = [
         ("https://jaiminisbox.com/reader/read/uratarou/en/0/1/", {
-            "url": "f021de7f31ee3a3f688fdf3e8183aef4226c2b50",
             "keyword": "d187df3e3b6dbe09ec163626f6fd7c57133ab163",
         }),
         ("https://jaiminisbox.com/reader/read/dr-stone/en/0/16/", {

gallery_dl/extractor/seiga.py

@@ -74,8 +74,7 @@ class SeigaUserExtractor(SeigaExtractor):
                 r"user/illust/(\d+)")]
     test = [
         ("http://seiga.nicovideo.jp/user/illust/39537793", {
-            "keyword": "66b3309484417fb5e76b72d5bd64526fa5d9b6a3",
-            "content": "40dc3b454d429108cb834b9e449229231010ddfa",
+            "keyword": "15e4158164f9309c75c0f97169e0026b13a642ed",
         }),
         ("http://seiga.nicovideo.jp/user/illust/79433", {
             "url": "da39a3ee5e6b4b0d3255bfef95601890afd80709",