fixed error reporting in SkSL->Metal
fixed error reporting in SkSL->Metal

Bug: skia:8843
Change-Id: Ie27e805fb4fc1efd34253645cb82cc88f682884d
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/205926
Commit-Queue: Ethan Nicholas <d2cfb8c2f603266f4dc3a6f09acbcc0089603b37@google.com>
Commit-Queue: Kevin Lubick <7cdab2cfab351f23814786ba39716e90eed69047@google.com>
Auto-Submit: Ethan Nicholas <d2cfb8c2f603266f4dc3a6f09acbcc0089603b37@google.com>
Reviewed-by: Kevin Lubick <7cdab2cfab351f23814786ba39716e90eed69047@google.com>

aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,rubenvb/skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,HalCanary/skia-hc,rubenvb/skia,aosp-mirror/platform_external_skia,HalCanary/skia-hc,HalCanary/skia-hc,aosp-mirror/platform_external_skia,rubenvb/skia,HalCanary/skia-hc,HalCanary/skia-hc,aosp-mirror/platform_external_skia,google/skia,google/skia,google/skia,HalCanary/skia-hc,HalCanary/skia-hc,rubenvb/skia,HalCanary/skia-hc,rubenvb/skia,google/skia,HalCanary/skia-hc,HalCanary/skia-hc,rubenvb/skia,google/skia,rubenvb/skia,rubenvb/skia,aosp-mirror/platform_external_skia,google/skia,rubenvb/skia,google/skia,google/skia,aosp-mirror/platform_external_skia,google/skia,rubenvb/skia,google/skia