Add a new zErrMsg field to the sqlite3_vtab structure to support returning

Add a new zErrMsg field to the sqlite3_vtab structure to support returning
error messages from virtual table constructors.  This change means that
virtual table implementations compiled as loadable extensions for version
3.3.7 will need to be recompile for version 3.3.8 and will not be usable
by both versions at one.  The virtual table mechanism is still considered
experimental so we feel justified in breaking backwards compatibility
in this way.  Additional interface changes might occurs in the future. (CVS 3401)

FossilOrigin-Name: 36693a5cb72b4363010f9ab0866e1f7865f65275