added WithFailOnPatternNotExist

README.md

@@ -120,6 +120,17 @@ that if the glob pattern references a path that does not exist (such as
 `nonexistent/path/*`), this is _not_ considered an IO error: it is considered a
 pattern with no matches.
 
+```go
+WithFailOnPatternNotExist()
+```
+
+If passed, doublestar will abort and return `doublestar.ErrPatternNotExist` if
+the pattern references a path that does not exist before any meta characters
+such as `nonexistent/path/*`. Note that alts (ie, `{...}`) are expanded before
+this check. In other words, a pattern such as `{a,b}/*` may fail if either `a`
+or `b` do not exist but `*/{a,b}` will never fail because the star may match
+nothing.
+
 ### Glob
 
 ```go

doublestar.go

@@ -1,8 +1,13 @@
 package doublestar
 
 import (
+	"errors"
 	"path"
 )
 
 // ErrBadPattern indicates a pattern was malformed.
 var ErrBadPattern = path.ErrBadPattern
+
+// ErrPatternNotExist indicates that the pattern passed to Glob, GlobWalk, or
+// FilepathGlob references a path that does not exist.
+var ErrPatternNotExist = errors.New("pattern does not exist")