Avoid throwing plain RuntimeException (plus related polishing)

See gh-24805

spring-core/src/main/java/org/springframework/util/StreamUtils.java

@@ -97,8 +97,6 @@ public abstract class StreamUtils {
 	/**
 	 * Copy the contents of the given {@link ByteArrayOutputStream} into a {@link String}.
 	 * <p>This is a more effective equivalent of {@code new String(baos.toByteArray(), charset)}.
-	 * <p>As long as the {@code charset} is already available at the point of
-	 * invocation, no exception is expected to be thrown by this method.
 	 * @param baos the {@code ByteArrayOutputStream} to be copied into a String
 	 * @param charset the {@link Charset} to use to decode the bytes
 	 * @return the String that has been copied to (possibly empty)
@@ -108,10 +106,12 @@ public abstract class StreamUtils {
 		Assert.notNull(baos, "No ByteArrayOutputStream specified");
 		Assert.notNull(charset, "No Charset specified");
 		try {
+			// Can be replaced with toString(Charset) call in Java 10+
 			return baos.toString(charset.name());
 		}
 		catch (UnsupportedEncodingException ex) {
-			throw new RuntimeException("Failed to copy contents of ByteArrayOutputStream into a String", ex);
+			// Should never happen
+			throw new IllegalArgumentException("Invalid charset name: " + charset, ex);
 		}
 	}
 
@@ -261,6 +261,7 @@ public abstract class StreamUtils {
 		return new NonClosingOutputStream(out);
 	}
 
+
 	private static class NonClosingInputStream extends FilterInputStream {
 
 		public NonClosingInputStream(InputStream in) {