Make conversion error clearer and less detailed.

[kuba--]

Signed-off-by: kuba-- [email protected]
Closes src-d/gitbase#615

[jfontan]

It would be nice to have the expression that fails to be casted but I suppose you don't have that information at that point.

[kuba--]

I have it on the higher level, but not here. On the other hand on the higher level it's hard to figure out what is the error, because it comes from spf13/cast. Unfortunately in this package error handling is not done very well - errors are either hardcoded/formatted strings or private vars. That's why here:

go-mysql-server/sql/expression/convert.go

Line 187 in 5902fdb

if err.Error() == "unable to cast negative value" {

we have to do this ugly thing!