Fix StrError, move Format*ErrorMessage to the implementation.

format.h

@@ -490,20 +490,15 @@ class UTF16ToUTF8 {
 #endif
 
 // Portable thread-safe version of strerror.
-// Returns a pointer to a string describing the error code. This can be
-// either a pointer to a string stored in buffer, or a pointer to some
-// static immutable string. Sets errno to one of the following values:
+// Sets buffer to point to a string describing the error code.
+// This can be either a pointer to a string stored in buffer,
+// or a pointer to some static immutable string.
+// Returns one of the following values:
 //   0      - success
 //   ERANGE - buffer is not large enough to store the error message
 //   other  - failure
 // Buffer should be at least of size 1.
-char *StrError(int error_code, char *buffer, std::size_t buffer_size);
-
-// Formats a standard C library error message writing the output to out.
-void FormatCErrorMessage(Writer &out, int error_code, StringRef message);
-
-// Formats a system error message writing the output to out.
-void FormatSystemErrorMessage(Writer &out, int error_code, StringRef message);
+int StrError(int error_code, char *&buffer, std::size_t buffer_size);
 
 }  // namespace internal