Fetching URLs in a large dataset ends with an OutOfMemoryError with no feedback for the user

Describe the bug
A fetching URL's operation on a large dataset runs out of memory and gets stuck on 99%, giving no feedback to the user.

To Reproduce
Steps to reproduce the behavior:
Run an "Add column by fetching URL's" operation on ca 30k rows.

Current Results
Once the indicator on top of the page gets to 98 or 99%, it gets stuck there forever, giving no feedback to the user that something went wrong. When I check the console, I see:

10:38:08.848 [        project_utilities] Saved project '2337537703446' (2331ms)
Exception in thread "Thread-8" com.google.common.util.concurrent.ExecutionError: java.lang.OutOfMemoryError: GC overhead limit exceeded
	at com.google.common.cache.LocalCache$Segment.get(LocalCache.java:2205)
	at com.google.common.cache.LocalCache.get(LocalCache.java:3953)
	at com.google.common.cache.LocalCache.getOrLoad(LocalCache.java:3957)
	at com.google.common.cache.LocalCache$LocalLoadingCache.get(LocalCache.java:4875)
	at com.google.refine.operations.column.ColumnAdditionByFetchingURLsOperation$ColumnAdditionByFetchingURLsProcess.cachedFetch(ColumnAdditionByFetchingURLsOperation.java:331)
	at com.google.refine.operations.column.ColumnAdditionByFetchingURLsOperation$ColumnAdditionByFetchingURLsProcess.run(ColumnAdditionByFetchingURLsOperation.java:292)
	at java.lang.Thread.run(Thread.java:748)
Caused by: java.lang.OutOfMemoryError: GC overhead limit exceeded

Expected behavior
Either not run out of memory ;) or at least show an error message in the interface.

Desktop (please complete the following information):

• OS: Ubuntu 18.04.3 LTS
• Browser Version: Chrome Version 78.0.3904.87 (Official Build) (64-bit)
• JRE or JDK Version:
  openjdk version "1.8.0_232"
  OpenJDK Runtime Environment (build 1.8.0_232-8u232-b09-0ubuntu1~18.04.1-b09)
  OpenJDK 64-Bit Server VM (build 25.232-b09, mixed mode)

OpenRefine (please complete the following information):

• Version 3.3 Beta

Start with ColumnAdditionByFetchingURLsOperation.java, especially cachedFetch and run, using the stack trace and the 30k-row reproduction as entry points. Investigate how the operation reports failures to the interface; done means the large fetch either avoids the OutOfMemoryError or presents an error instead of remaining stuck at 98–99%.