Clean up the return type of parser_t::eval

parser_t::eval indicates whether there was a parse error. It can be
easily confused with the status of the execution. Use a real type to
make it more clear.

src/parser.h

@@ -53,6 +53,18 @@ enum class loop_status_t {
     continues,  /// current loop block should be skipped
 };
 
+/// Result of the source code form of eval.
+enum class eval_result_t {
+    /// eval was able to evaluate the source or tree.
+    ok,
+
+    /// Evaluation was cancelled, e.g. because of a signal.
+    cancelled,
+
+    /// Parse or execution error (but not simply a failed command).
+    error,
+};
+
 /// block_t represents a block of commands.
 class block_t {
     /// Construct from a block type.
@@ -258,18 +270,18 @@ class parser_t : public std::enable_shared_from_this<parser_t> {
     /// \param io io redirections to perform on all started jobs
     /// \param block_type The type of block to push on the block stack
     ///
-    /// \return 0 on success, 1 on a parse error.
-    int eval(wcstring cmd, const io_chain_t &io, enum block_type_t block_type);
+    /// \return the eval result,
+    eval_result_t eval(const wcstring &cmd, const io_chain_t &io, enum block_type_t block_type);
 
     /// Evaluate the parsed source ps.
-    /// \return 0 on success, 1 on a parse error.
-    int eval(parsed_source_ref_t ps, const io_chain_t &io, enum block_type_t block_type);
+    /// Because the source has been parsed, a syntax error is impossible.
+    eval_result_t eval(parsed_source_ref_t ps, const io_chain_t &io, enum block_type_t block_type);
 
     /// Evaluates a node.
     /// The node type must be grammar::statement or grammar::job_list.
     template <typename T>
-    int eval_node(parsed_source_ref_t ps, tnode_t<T> node, block_type_t block_type,
-                  job_lineage_t lineage);
+    eval_result_t eval_node(parsed_source_ref_t ps, tnode_t<T> node, block_type_t block_type,
+                            job_lineage_t lineage);
 
     /// Evaluate line as a list of parameters, i.e. tokenize it and perform parameter expansion and
     /// cmdsubst execution on the tokens. Errors are ignored. If a parser is provided, it is used