Polish a few things (#67)

* Improve some match statements using macros

* Use `thiserror` instead of manually impl'ing Error

Cargo.lock

@@ -60,6 +60,7 @@ dependencies = [
  "graphite-document-core",
  "graphite-proc-macros",
  "log",
+ "thiserror",
 ]
 
 [[package]]
@@ -155,6 +156,26 @@ dependencies = [
  "unicode-xid",
 ]
 
+[[package]]
+name = "thiserror"
+version = "1.0.24"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "e0f4a65597094d4483ddaed134f409b2cb7c1beccf25201a9f73c719254fa98e"
+dependencies = [
+ "thiserror-impl",
+]
+
+[[package]]
+name = "thiserror-impl"
+version = "1.0.24"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "7765189610d8241a44529806d6fd1f2e0a08734313a35d5b3a556f92b381f3c0"
+dependencies = [
+ "proc-macro2",
+ "quote",
+ "syn",
+]
+
 [[package]]
 name = "unicode-xid"
 version = "0.2.1"