improve ux for relocating partial uploads

if someone accidentally starts uploading a file in the wrong folder,
it was not obvious that you can forget that upload in the unpost tab

this '(explain)' button in the upload-error hopefully explains that,
and upload immediately commences when the initial attempt is aborted

on the backend, cleanup the dupesched when an upload is
aborted, and save some cpu by adding unique entries only

scripts/tl.js

@@ -627,6 +627,7 @@ var tl_browser = {
 		"u_hashdone": 'hashing done',
 		"u_hashing": 'hash',
 		"u_hs": 'handshaking...',
+		"u_started": "the files are now being uploaded; see [🚀]",
 		"u_dupdefer": "duplicate; will be processed after all other files",
 		"u_actx": "click this text to prevent loss of<br />performance when switching to other windows/tabs",
 		"u_fixed": "OK!&nbsp; Fixed it 👍",
@@ -662,6 +663,7 @@ var tl_browser = {
 		"ue_la": 'you are currently logged in as "{0}"',
 		"ue_sr": 'you are currently in file-search mode\n\nswitch to upload-mode by clicking the magnifying glass 🔎 (next to the big SEARCH button), and try uploading again\n\nsorry',
 		"ue_ta": 'try uploading again, it should work now',
+		"ue_ab": "this file is already being uploaded into another folder, and that upload must be completed before the file can be uploaded elsewhere.\n\nYou can abort and forget the initial upload using the top-left 🧯",
 		"ur_1uo": "OK: File uploaded successfully",
 		"ur_auo": "OK: All {0} files uploaded successfully",
 		"ur_1so": "OK: File found on server",