Improve error message for unresolved toolchains

src/main/java/com/google/devtools/build/lib/skyframe/ToolchainResolutionFunction.java

@@ -50,6 +50,7 @@
 import java.util.List;
 import java.util.Map;
 import java.util.Optional;
+import java.util.regex.Pattern;
 import java.util.Set;
 import java.util.stream.Collectors;
 import javax.annotation.Nullable;
@@ -570,20 +571,13 @@ protected Code getDetailedCode() {
     }
 
     private static String getMessage(List<Label> missingToolchainTypes) {
-      if (missingToolchainTypes.size() == 1
-          && Iterables.getOnlyElement(missingToolchainTypes)
-              .toString()
-              .equals("@bazel_tools//tools/cpp:toolchain_type")) {
-        return "No matching toolchains found for types @bazel_tools//tools/cpp:toolchain_type. "
-            + "Maybe --incompatible_use_cc_configure_from_rules_cc has been flipped and there "
-            + "is no default C++ toolchain added in the WORKSPACE file? See "
-            + "https://github.com/bazelbuild/bazel/issues/10134 for details and migration "
-            + "instructions.";
-      }
-
+      List<String> labelStrings =
+          missingToolchainTypes.stream().map(Label::toString).collect(Collectors.toList());
       return String.format(
-          "no matching toolchains found for types %s",
-          missingToolchainTypes.stream().map(Label::toString).collect(joining(", ")));
+          "No matching toolchains found for types %s. "
+          + "To debug, rerun with --toolchain_resolution_debug=%s",
+          String.join(", ", labelStrings),
+          labelStrings.stream().map(Pattern::quote).collect(joining("|")));
     }
   }