stage2: introduce Module.failed_root_source_file

Use case:

zig build-exe non_existent_file.zig

Previous behavior:

error.FileNotFound, followed by an error return trace

Behavior after this commit:

error: unable to read non_existent_file.zig: FileNotFound
(end of stderr, exit code 1)

This turns AllErrors.Message into a tagged union which now has the
capability to represent both "plain" errors as well as source-based
errors (with file, line, column, byte offset). The "no entry point found"
error has moved to be a plain error message.

src/Module.zig

@@ -78,6 +78,9 @@ import_table: std.StringArrayHashMapUnmanaged(*Scope.File) = .{},
 /// previous analysis.
 generation: u32 = 0,
 
+/// When populated it means there was an error opening/reading the root source file.
+failed_root_src_file: ?anyerror = null,
+
 stage1_flags: packed struct {
     have_winmain: bool = false,
     have_wwinmain: bool = false,