sql: add OP_SetDiag opcode in VDBE

To separate the error setting and execution halting, a new opcode
OP_SetDiag was created. The only functionality of the opcode is
the execution of diag_set(). It is important to note that
OP_SetDiag does not set is_aborted to true, so we can continue
working with other opcodes, if necessary. This function allows us
to perform cleanup in some special cases, for example, when
creating a constraint failed because of the creation of two or
more constraints with the same name in the same CREATE TABLE
statement.

Since now diag_set() is executed in OP_SetDiag, this functionality
has been removed from OP_Halt.

Needed for #4183