[Enhanced enums] CFE error report wording issues

[sgrekhov]

Please see the following co19 test https://github.com/dart-lang/co19/blob/master/LanguageFeatures/Enhanced-Enum/semantics_A10_t03.dart

CFE prodices the following error on this test.

/===================================================================================================================================\
| cfe-strong-linux:co19/LanguageFeatures/Enhanced-Enum/semantics_A10_t03 is new and failed (MissingCompileTimeError, expected Pass) |
\===================================================================================================================================/

--- Command "fasta" (took 136ms):
(cd /b/s/w/ir/ ; DART_CONFIGURATION=ReleaseX64 ./out/ReleaseX64/dart ./pkg/front_end/tool/_fasta/compile.dart --verify --skip-platform-verification -o ./out/ReleaseX64/generated_compilations/cfe-strong-linux/tests_co19_src_LanguageFeatures_Enhanced-Enum_semantics_A10_t03/out.dill --platform ./out/ReleaseX64/vm_platform_strong.dill -Dtest_runner.configuration=cfe-strong-linux --enable-experiment=enhanced-enums --nnbd-strong --packages=./.packages ./tests/co19/src/LanguageFeatures/Enhanced-Enum/semantics_A10_t03.dart )

static error failures:
- Unexpected error at line 31, column 6: Applying the mixin 'M1' to '_Enum' introduces an erroneous override of 'index'.

- Unexpected error at line 31, column 6: Applying the mixin 'M1' to '_Enum' introduces an erroneous override of 'toString'.

- Unexpected error at line 31, column 6: The return type of the method 'E1.toString' is 'String', which does not match the return type, 'int', of the overridden method, '_Enum with M1.toString'.

- Unexpected error at line 49, column 6: Can't declare a member that conflicts with an inherited one.

- Unexpected error at line 49, column 6: Can't inherit members that conflict with each other.

--- Re-run this test:
python3 tools/test.py -n cfe-strong-linux co19/LanguageFeatures/Enhanced-Enum/semantics_A10_t03

• The error message should probably not mention _Enum.
• The override errors for index and toString are reported correctly, but on E1 where M1 is applied, and they could be reported in M1 (there is no way M1 could be successfully applied to any superclass).
• The errors on line 49 are not informative.