Improve mapper resource failure diagnostics

[DragonFSKY]

Summary

• Report mapper resource opening failures separately from XML parsing failures.
• Suggest resolving wildcard locations with ResourcePatternResolver.getResources() before calling setMapperLocations(...).
• Preserve the existing parse-failure diagnostic and ErrorContext reset behavior.

Closes #792

Testing

• ./mvnw verify (225 tests, 0 failures, 0 errors, 15 skipped)
• git diff --check

[coveralls]

coverage: 90.616% (+0.2%) from 90.381% — DragonFSKY:fix/issue-792-mapper-resource-diagnostics into mybatis:master