Renamed error -> err

- enum lfs3_error -> enum lfs3_err
- err -> err

Really this just updates `enum lfs3_err` to match the prefixes used
everywhere else. And because enum types are kind of useless in C, this
has no effect on any other part of the codebase.

lfs3.h

@@ -72,7 +72,7 @@ typedef int32_t  lfs3_sdid_t;
 
 // Possible error codes, these are negative to allow
 // valid positive return values
-enum lfs3_error {
+enum lfs3_err {
     LFS3_ERR_OK          = 0,    // No error
     LFS3_ERR_UNKNOWN     = -1,   // Unknown error
     LFS3_ERR_INVAL       = -22,  // Invalid parameter