shell: fix syntax error + autocompletion exception

Somehow there were two types of syntax errors, and the autocompletion
code was not catching the correct one.

shell/i.cpp

@@ -443,7 +443,8 @@ marked as part of functions, data, interrupt handlers, etc.
 )");
 
 static ShellCommand _if_cmd(
-    "if", [](Session &s, Parser &p) {
+    "if",
+    [](Session &s, Parser &p) {
         auto args = parse_if(s, p);
         _if(s, args);
     },

shell/lexer.l

@@ -94,7 +94,7 @@ static bool lex_pop()
 }
 
 /* Error messages and exceptions */
-static void err(char const *format, ...)
+void lex_err(char const *format, ...)
 {
 	static char buf[1024];
 
@@ -103,8 +103,13 @@ static void err(char const *format, ...)
 	vsnprintf(buf, 1024, format, args);
 	va_end(args);
 
-	Input const &in = lex_current_input();
-	throw SyntaxError(in.filename.c_str(), in.line, buf);
+	if(lex_idle()) {
+		throw SyntaxError("(idle)", 0, buf);
+	}
+	else {
+		Input const &in = lex_current_input();
+		throw SyntaxError(in.filename.c_str(), in.line, buf);
+	}
 }
 
 /* Parse numerical values */
@@ -194,9 +199,9 @@ space        [ \t]+
 <*>"?"       { return '?'; }
 
  /* Generic error and word boundaries violations */
-<*>{syscall}{letter}  { err("invalid syscall number '%s'", yytext); }
-<*>{num}{letter}      { err("invalid numerical value '%s'", yytext); }
-<*>.                  { err("invalid token near '%s'", yytext); }
+<*>{syscall}{letter}  { lex_err("invalid syscall number '%s'", yytext); }
+<*>{num}{letter}      { lex_err("invalid numerical value '%s'", yytext); }
+<*>.                  { lex_err("invalid token near '%s'", yytext); }
 <<EOF>>               { if(lex_pop()) return T::END; }
 
 %%

shell/main.cpp

@@ -15,6 +15,7 @@
 #include "shell.h"
 #include "theme.h"
 #include "parser.h"
+#include "errors.h"
 #include "commands.h"
 #include <fxos/util/log.h>
 #include <fxos/semantics.h>
@@ -106,7 +107,7 @@ Parser::CompletionRequest parse_until_autocomplete(
     catch(Parser::CompletionRequest r) {
         return r;
     }
-    catch(Parser::SyntaxError &e) {
+    catch(SyntaxError &e) {
         /* Ignore syntax errors, just don't autocomplete */
     }
 

shell/parser.cpp

@@ -107,7 +107,7 @@ void Parser::end()
 {
     m_options.clear();
     if(!at_end())
-        throw SyntaxError("expected end of command");
+        lex_err("expected end of command");
 }
 
 void Parser::exhaust()
@@ -166,16 +166,15 @@ Token Parser::expect(std::initializer_list<T> types, bool ignore_spaces)
 
     if(!correct_type) {
         static char err[128];
-        int offset = sprintf(err, "expected ");
+        int offset = 0;
 
         for(auto it = types.begin(); it != types.end(); it++) {
             offset += sprintf(err + offset, "%s%s%s",
                 (it != types.begin() && it + 1 == types.end() ? "or " : ""),
-                (*it).str().c_str(), (it + 1 == types.end() ? "; " : ", "));
+                (*it).str().c_str(), (it + 1 == types.end() ? "" : ", "));
         }
 
-        sprintf(err + offset, "instead found %s", m_la.str().c_str());
-        throw SyntaxError(err);
+        lex_err("expected %s; instead found %s", err, m_la.str().c_str());
     }
 
     Token t = feed(ignore_spaces);

shell/parser.h

@@ -95,6 +95,8 @@ void lex_repl(std::string input);
 /* Lex the input coming from these files, sequentially */
 void lex_include(std::vector<std::string> files);
 
+/* For the parser: format and throw a syntax error at current location */
+void lex_err(char const *format, ...);
 /* For the parser: get one token from the current source */
 Token lex_read();
 
@@ -166,12 +168,6 @@ public:
 	// Completion system
 	//---
 
-	class SyntaxError: public std::invalid_argument
-	{
-	public:
-		SyntaxError(char const *what): std::invalid_argument(what) {}
-	};
-
 	/* A completion request is thrown by the parser when in completion mode to
 	   signal the expected next element */
 	struct CompletionRequest