Bump github.com/pkg/errors from 0.8.1 to 0.9.1 (#40)

Bumps [github.com/pkg/errors](https://github.com/pkg/errors) from 0.8.1 to 0.9.1.
- [Release notes](https://github.com/pkg/errors/releases)
- [Commits](pkg/errors@v0.8.1...v0.9.1)

Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

go.mod

@@ -10,7 +10,7 @@ require (
 	github.com/facebookgo/subset v0.0.0-20150612182917-8dac2c3c4870 // indirect
 	github.com/gogo/protobuf v1.3.1
 	github.com/jmhodges/levigo v1.0.0
-	github.com/pkg/errors v0.8.1
+	github.com/pkg/errors v0.9.1
 	github.com/stretchr/testify v1.4.0
 	github.com/syndtr/goleveldb v1.0.1-0.20190923125748-758128399b1d
 	github.com/tecbot/gorocksdb v0.0.0-20191017175515-d217d93fd4c5

go.sum

@@ -43,6 +43,8 @@ github.com/onsi/gomega v1.4.3 h1:RE1xgDvH7imwFD45h+u2SgIfERHlS2yNG4DObb5BSKU=
 github.com/onsi/gomega v1.4.3/go.mod h1:ex+gbHU/CVuBBDIJjb2X0qEXbFg53c61hWP/1CpauHY=
 github.com/pkg/errors v0.8.1 h1:iURUrRGxPUNPdy5/HRSm+Yj6okJ6UtLINN0Q9M4+h3I=
 github.com/pkg/errors v0.8.1/go.mod h1:bwawxfHBFNV+L2hUp1rHADufV3IMtnDRdf1r5NINEl0=
+github.com/pkg/errors v0.9.1 h1:FEBLx1zS214owpjy7qsBeixbURkuhQAwrK5UwLGTwt4=
+github.com/pkg/errors v0.9.1/go.mod h1:bwawxfHBFNV+L2hUp1rHADufV3IMtnDRdf1r5NINEl0=
 github.com/pmezard/go-difflib v1.0.0 h1:4DBwDE0NGyQoBHbLQYPwSUPoCMWR5BEzIk/f1lZbAQM=
 github.com/pmezard/go-difflib v1.0.0/go.mod h1:iKH77koFhYxTK1pcRnkKkqfTogsbg7gZNVY4sRDYZ/4=
 github.com/prometheus/client_model v0.0.0-20190812154241-14fe0d1b01d4/go.mod h1:xMI15A0UPsDsEKsMN9yxemIoYk6Tm2C1GtYGdfGttqA=