Warn when building assets from files if some referenced contexts are missing

Cargo.lock

@@ -381,9 +381,9 @@ dependencies = [
 
 [[package]]
 name = "fancy-regex"
-version = "0.3.5"
+version = "0.7.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "ae91abf6555234338687bb47913978d275539235fcb77ba9863b779090b42b14"
+checksum = "9d6b8560a05112eb52f04b00e5d3790c0dd75d9d980eb8a122fb23b92a623ccf"
 dependencies = [
  "bit-set",
  "regex",
@@ -1088,9 +1088,9 @@ dependencies = [
 
 [[package]]
 name = "syntect"
-version = "4.5.0"
+version = "4.6.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "2bfac2b23b4d049dc9a89353b4e06bbc85a8f42020cccbe5409a115cf19031e5"
+checksum = "8b20815bbe80ee0be06e6957450a841185fcf690fe0178f14d77a05ce2caa031"
 dependencies = [
  "bincode",
  "bitflags",