I can take a stab at this.
Improve error message for incompatible_string_join_requires_strings
When the flag `--incompatible_string_join_requires_strings` is enabled, Baze checks that each element is a string (see https://github.com/bazelbuild/bazel/issues/7802).

However, the error message is just a type error. This can be confusing for users who are upgrading Bazel. The error message should at least include the name of the flag.

Since we're flipping the flag in Bazel 0.27, it would be great to improve the error message before that. @jin, @c-parsons, do you have time to do it this week? (tomorrow is public holiday in Germany)

Thanks!
src/main/java/com/google/devtools/build/lib/syntax/StringModule.java
src/main/java/com/google/devtools/build/lib/syntax/StringModule.java
diff --git a/src/main/java/com/google/devtools/build/lib/syntax/StringModule.java b/src/main/java/com/google/devtools/build/lib/syntax/StringModule.java
index 1d4cf1bca70c4f..4e2e46ba5a12be 100644
--- a/src/main/java/com/google/devtools/build/lib/syntax/StringModule.java
+++ b/src/main/java/com/google/devtools/build/lib/syntax/StringModule.java
@@ -115,7 +115,11 @@ public String join(String self, SkylarkList<?> elements, Location loc, Environme
         if (!(item instanceof String)) {
           throw new EvalException(
               loc,
-              "sequence element must be a string (got '" + EvalUtils.getDataTypeName(item) + "')");
+              "sequence element must be a string (got '"
+                  + EvalUtils.getDataTypeName(item)
+                  + "'). See https://github.com/bazelbuild/bazel/issues/7802 for information about "
+                  + "--incompatible_string_join_requires_strings."
+          );
         }
       }
     }
