fix: include string errors during convert

[shetzel]

What does this PR do?

Adds the error message to the new error when the caught error is a String.

What issues does this PR fix or reference?

forcedotcom/cli#1420
@W-12099973@

Functionality Before

sfdx force:source:deploy -p force-app/main/default/classes/PagedResult.cls
*** Deploying with SOAP API v56.0 ***
ERROR running force:source:deploy:  Component conversion failed: undefined

Functionality After

sfdx force:source:deploy -p force-app/main/default/classes/PagedResult.cls
*** Deploying with SOAP API v56.0 ***
ERROR running force:source:deploy:  Component conversion failed: error converting stream - Error: ENOENT: no such file or directory, open '.../ebikes-lwc/force-app/main/default/classes/PagedResult.cls-meta.xml'

[mshanemc]

QA:

✅ looks exactly like the PR says it does.

note: the absence of a meta file, when deploying a whole folder or project, just omits the file from the deployment.