Refactor: Enhance error handling and readability per review comments

Author: nbayati

oauth2_http/java/com/google/auth/mtls/X509Provider.java

@@ -98,7 +98,7 @@ public String getCertificatePath() throws IOException {
     String certPath = loadedConfig.getCertPath();
     if (Strings.isNullOrEmpty(certPath)) {
       // Ensure the loaded configuration actually contains the required path.
-      throw new IOException(
+      throw new CertificateSourceUnavailableException(
           "Certificate configuration loaded successfully, but does not contain a 'certificate_file' path.");
     }
     return certPath;

oauth2_http/java/com/google/auth/oauth2/IdentityPoolCredentialSource.java

@@ -172,7 +172,7 @@ public static class CertificateConfig implements java.io.Serializable {
           certificateConfigLocation != null && !certificateConfigLocation.isEmpty();
 
       checkArgument(
-          !(!useDefault && !locationIsPresent),
+          (useDefault || locationIsPresent),
           "credentials: \"certificate\" object must either specify a certificate_config_location or use_default_certificate_config should be true");
 
       checkArgument(