Add QL library for working with Go frontend errors.

ql/src/go.qll

@@ -7,6 +7,7 @@ import semmle.go.AST
 import semmle.go.Comments
 import semmle.go.Concepts
 import semmle.go.Decls
+import semmle.go.Errors
 import semmle.go.Expr
 import semmle.go.Files
 import semmle.go.GoMod

ql/src/semmle/go/Errors.qll

@@ -0,0 +1,50 @@
+/** Provides classes for working with Go frontend errors recorded during extraction. */
+
+import go
+
+/**
+ * An error reported by the Go frontend during extraction.
+ */
+class Error extends @error {
+  /** Gets the message associated with this error. */
+  string getMessage() { errors(this, _, result, _, _, _, _, _, _) }
+
+  /** Gets the raw position reported by the frontend for this error. */
+  string getRawPosition() { errors(this, _, _, result, _, _, _, _, _) }
+
+  /** Gets the package in which this error was reported. */
+  Package getPackage() { errors(this, _, _, _, _, _, _, result, _) }
+
+  /** Gets the index of this error among all errors reported for the same package. */
+  int getIndex() { errors(this, _, _, _, _, _, _, _, result) }
+
+  /**
+   * Holds if this element is at the specified location.
+   * The location spans column `startcolumn` of line `startline` to
+   * column `endcolumn` of line `endline` in file `filepath`.
+   * For more information, see
+   * [LGTM locations](https://lgtm.com/help/ql/locations).
+   */
+  predicate hasLocationInfo(
+    string filepath, int startline, int startcolumn, int endline, int endcolumn
+  ) {
+    errors(this, _, _, _, filepath, startline, startcolumn, _, _) and
+    endline = startline and
+    endcolumn = startcolumn
+  }
+
+  /** Gets a textual representation of this error. */
+  string toString() { result = getMessage() }
+}
+
+/** An error reported by an unknown part of the Go frontend. */
+class UnknownError extends Error, @unknownerror { }
+
+/** An error reported by the Go frontend driver. */
+class ListError extends Error, @listerror { }
+
+/** An error reported by the Go parser. */
+class ParseError extends Error, @parseerror { }
+
+/** An error reported by the Go type checker. */
+class TypeError extends Error, @typeerror { }