Restructure project directories (#333)

`/client/web` -> `/frontend`
`/client/cli` -> *delete for now*
`/client/native` -> *delete for now*
`/core/editor` -> `/editor`
`/core/document` -> `/graphene`
`/core/renderer` -> `/charcoal`
`/core/proc-macro` -> `/proc-macros` *(now plural)*

editor/src/misc/error.rs

@@ -0,0 +1,35 @@
+use crate::Color;
+use graphene::DocumentError;
+use thiserror::Error;
+
+/// The error type used by the Graphite editor.
+#[derive(Clone, Debug, Error)]
+pub enum EditorError {
+	#[error("Failed to execute operation: {0}")]
+	InvalidOperation(String),
+	#[error("{0}")]
+	Misc(String),
+	#[error("Tried to construct an invalid color {0:?}")]
+	Color(String),
+	#[error("The requested tool does not exist")]
+	UnknownTool,
+	#[error("The operation caused a document error {0:?}")]
+	Document(String),
+	#[error("A Rollback was initated but no transaction was in progress")]
+	NoTransactionInProgress,
+}
+
+macro_rules! derive_from {
+	($type:ty, $kind:ident) => {
+		impl From<$type> for EditorError {
+			fn from(error: $type) -> Self {
+				EditorError::$kind(format!("{:?}", error))
+			}
+		}
+	};
+}
+
+derive_from!(&str, Misc);
+derive_from!(String, Misc);
+derive_from!(Color, Color);
+derive_from!(DocumentError, Document);