ToolchainTypeLookupUtil swallows error for missing targets

[katre]

If a target used as a toolchain type does not exist, we get a generic error message:

ERROR: While resolving toolchains for target //:sample: Target //subpackage:toolchain_type was referenced as a toolchain type, but does not provide ToolchainTypeInfo
WARNING: errors encountered while analyzing target '//:sample': it will not be built

Instead we should get specific error information about the fact that (in this case) //:toolchain_type does not exist.