Replace () error types in several functions.

[tmccombs]

This is an initial pass at #299.

It does not change ParseError to the more idiomatic Error name, or
change with_default_port's return type.

I'm also not 100% sure that the errors I returned are correct.

---

This change is 

[bors-servo]

☔ The latest upstream changes (presumably #517) made this pull request unmergeable. Please resolve the merge conflicts.

[opilar]

@tmccombs seems like you forgot to update windows specific function: file_url_segments_to_pathbuf_windows.

[opilar]

Remaining fixes:

diff --git a/src/lib.rs b/src/lib.rs
index f87fb02..6690e82 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -2408,7 +2408,7 @@ fn file_url_segments_to_pathbuf(
 fn file_url_segments_to_pathbuf(
     host: Option<&str>,
     segments: str::Split<char>,
-) -> Result<PathBuf, ()> {
+) -> Result<PathBuf, ParseError> {
     file_url_segments_to_pathbuf_windows(host, segments)
 }
 
@@ -2417,16 +2417,16 @@ fn file_url_segments_to_pathbuf(
 fn file_url_segments_to_pathbuf_windows(
     host: Option<&str>,
     mut segments: str::Split<char>,
-) -> Result<PathBuf, ()> {
+) -> Result<PathBuf, ParseError> {
     let mut string = if let Some(host) = host {
         r"\\".to_owned() + host
     } else {
-        let first = segments.next().ok_or(())?;
+        let first = segments.next().ok_or(ParseError::InvalidLocalPath)?;
 
         match first.len() {
             2 => {
                 if !first.starts_with(parser::ascii_alpha) || first.as_bytes()[1] != b':' {
-                    return Err(());
+                    return Err(ParseError::InvalidLocalPath);
                 }
 
                 first.to_owned()
@@ -2434,17 +2434,17 @@ fn file_url_segments_to_pathbuf_windows(
 
             4 => {
                 if !first.starts_with(parser::ascii_alpha) {
-                    return Err(());
+                    return Err(ParseError::InvalidLocalPath);
                 }
                 let bytes = first.as_bytes();
                 if bytes[1] != b'%' || bytes[2] != b'3' || (bytes[3] != b'a' && bytes[3] != b'A') {
-                    return Err(());
+                    return Err(ParseError::InvalidLocalPath);
                 }
 
                 first[0..1].to_owned() + ":"
             }
 
-            _ => return Err(()),
+            _ => return Err(ParseError::InvalidLocalPath),
         }
     };
 
@@ -2454,7 +2454,7 @@ fn file_url_segments_to_pathbuf_windows(
         // Currently non-unicode windows paths cannot be represented
         match String::from_utf8(percent_decode(segment.as_bytes()).collect()) {
             Ok(s) => string.push_str(&s),
-            Err(..) => return Err(()),
+            Err(..) => return Err(ParseError::InvalidLocalPath),
         }
     }
     let path = PathBuf::from(string); 

[djc]

Changing the title to note that these changes are semver-incompatible (in my understanding). Tracking all proposed semver-incompatible changes in #627.