expr: Fine-tune parser error message for common typo.

expr: Fine-tune parser error message for common typo.

It's easy to type "=" in place of "==" in an expression but the expression
parser's error message was far from clear.  For multibit numeric fields,
it said:
    Explicit `!= 0' is required for inequality test of multibit field
    against 0.
For string fields, the parser treated such an expression as "<name> != 0"
and thus it said:
    String field <name> is not compatible with numeric constant.

This improves the error message in each case to:
    Syntax error at `=' expecting relational operator.
which I hope to be clear.

Signed-off-by: Ben Pfaff <blp@ovn.org>
Acked-by: Ryan Moats <rmoats@us.ibm.com>