chore: avoid trailing whitespace in error message

This messed with the tests, where my editor was chopping of trailing
whitespace. While I'm in here, don't capitalize and parenthesize to
make this line fit in better with the rest of our logging/errors.

Author: djc

src/errors.rs

@@ -205,7 +205,7 @@ fn component_unavailable_msg(cs: &[Component], manifest: &Manifest, toolchain: &
             if toolchain.starts_with("nightly") {
                 let _ = write!(
                     buf,
-                    "Sometimes not all components are available in any given nightly. "
+                    "(sometimes not all components are available in any given nightly)"
                 );
             }
         }
@@ -235,7 +235,7 @@ fn component_unavailable_msg(cs: &[Component], manifest: &Manifest, toolchain: &
             if toolchain.starts_with("nightly") {
                 let _ = write!(
                     buf,
-                    "Sometimes not all components are available in any given nightly. "
+                    "(sometimes not all components are available in any given nightly)"
                 );
             }
         }

tests/suite/cli_v2.rs

@@ -1972,7 +1972,7 @@ async fn add_missing_component() {
         .with_stderr(snapbox::str![[r#"
 ...
 error: component 'rls' for target '[HOST_TRIPLE]' is unavailable for download for channel 'nightly'
-Sometimes not all components are available in any given nightly. 
+(sometimes not all components are available in any given nightly)
 ...
 "#]])
         .is_err();